FIELD OF THE INVENTION The present invention relates to a solid titanium catalyst component used as a catalyst component for preparing olefin (co)polymers, a process for preparing the catalyst component, an olefin polymerization catalyst containing the catalyst component and an olefin polymerization process using the catalyst. BACKGROUND OF THE INVENTION Catalysts containing titanium compounds supported on active magnesium halides have hitherto been known as those used for preparing homopolymers of α-olefins or olefin copolymers such as ethylene/α-olefin copolymers. As the olefin polymerization catalysts, those comprising a solid titanium catalyst component consisting of magnesium, titanium, halogen and an electron donor and an organometallic compound catalyst component are known. It is also known that when the solid titanium catalyst component containing magnesium, titanium, halogen and an electron donor as its essential ingredients is used in the polymerization of a-olefins of 3 or more carbon atoms, polymers of high stereoregularity can be obtained in high yields. For preparing the solid titanium catalyst component, there is known, for example, a process wherein a hydrocarbon solution of a halogen-containing magnesium compound is contacted with a liquid titanium compound to form a solid product or a process wherein a hydrocarbon solution of a magnesium halide compound and a titanium compound is prepared and then a solid product is formed in the presence of an electron donor. As described above, many proposals relating to processes for preparing solid titanium catalyst components have been made, but studies of the properties of the resulting solid titanium catalyst components have been scarcely made. Under such circumstances as mentioned above, the present inventors studied to obtain a solid titanium catalyst component by the use of which olefin (co)polymers of high stereoregularity can be obtained with high polymerization activity. As a result, they have found that an olefin polymerization catalyst, which contains a solid titanium catalyst component having a specific microcrystalline size (size of microcrystals of magnesium halide constituting the solid titanium catalyst component), a specific volume of pores having a radius of not more than 0.1 µm, a specific volume of pores having a radius of 0.1 to 7.5 µm and a specific mean catalyst particle diameter, can prepare olefin (co)polymers with high polymerization activity. Moreover, they have also found that when α-olefins of 3 or more carbon atoms are polymerized in the presence of the catalyst, olefin (co)polymers of high stereoregularity can be obtained. Based on the finding, the present invention has been accomplished. OBJECT OF THE INVENTION The present invention has been made under such circumstances as described above, and it is an object of the invention to provide a solid titanium catalyst component capable of (co)polymerizing an olefin with high polymerization activity and a process for preparing the catalyst component. It is another object of the invention to provide an olefin polymerization catalyst containing the solid titanium catalyst component and an olefin polymerization process using the catalyst. SUMMARY OF THE INVENTION The solid titanium catalyst component for olefin polymerization according to the invention is a solid titanium catalyst component containing titanium, magnesium and halogen as its essential ingredients, wherein: (1) magnesium halide constituting the catalyst component has a microcrystalline size, as calculated from peak of the (110) face measured by X-ray diffractometry of the magnesium halide, of 3 to 100 A, (2) the volume of pores having a radius of not more than 0.1µm is not more than 0..20 cm3/g, (3) the volume of pores having a radius of 0.1 to 7.5 µm is not less than 0.30 cm3/g, and (4) the catalyst component has a mean particle diameter, as measured by a light transmission sedimentation method, of 0.5 to 80 µm. It is preferable that the solid titanium catalyst component for olefin polymerization according to the invention is a solid titanium catalyst component containing titanium, magnesium and halogen as its essential ingredients, wherein: (1) magnesium halide constituting the catalyst component has a microcrystalline size, as calculated from peak of the (110) face measured by X-ray diffractometry of the magnesium halide, of 10 to 40 Å, (2) the volume of pores having a radius of not more than 0.1 µm is not more than 0.01 cm3/g, (3) the volume of pores having a radius of 0.1 to 7.5 µm is not less than 0.50 cm3/g, and (4) the catalyst component has a mean particle diameter, as measured by a light transmission sedimentation method, of 0.5 to 80 µm . The solid titanium catalyst component can be prepared by contacting a magnesium compound in the liquid state with a liquid titanium compound in the presence of a diether compound having a fluorene ring, said diether compound being represented by the following formula (i): (Formula Removed) wherein Ra and Rb may be the same as or different from each other and are each an alkyl group of 1 to 6 carbon atoms, X and Y may be the same as or different from each other and are each an alkyl group of 1 to 6 carbon atoms or a halogen atom, m is a number of 0 < m < 4, and n is a number of 0 < n < 4. Further, the solid titanium catalyst component for olefin polymerization can be prepared by a process comprising the steps of: contacting a magnesium compound in the liquid state with a diether compound having a fluorene ring, said diether compound being represented by the above formula (i), and then contacting the resulting solution with a liquid titanium compound. The magnesium compound in the liquid state can be prepared by, for example, contacting a magnesium compound with a compound capable of solubilizing the magnesium compound and selected from the group consisting of alcohols, esters and ethers in a hydrocarbon solvent. The olefin polymerization catalyst according to the invention comprises: (A) the above-mentioned solid titanium catalyst component, (B) an organometallic compound catalyst component containing a metal selected from Group I to Group III of the periodic table, and (C) an electron donor. The olefin polymerization catalyst according to the invention may be a prepolymerized olefin-containing catalyst. The olefin polymerization process according to the invention comprises polymerizing or copolymerizing an olefin in the presence of the above-mentioned olefin polymerization catalyst. DETAILED DESCRIPTION OF THE INVENTION The solid titanium catalyst component for olefin polymerization, the process for preparing the catalyst component, the olefin polymerization catalyst containing the catalyst component and the olefin polymerization process using the catalyst according to the invention are described in detail hereinafter. The meaning of the term "polymerization" used herein is not limited to "homopolymerization" but may comprehend "copolymerization". Also, the meaning of the term "polymer" used herein is not limited to "homopolymer" but may comprehend "copolymer". (Solid titanium catalyst component) The solid titanium catalyst component for olefin polymerization according to the invention contains titanium, magnesium and halogen as its essential ingredients. In the solid titanium catalyst component for olefin polymerization, titanium is contained in an amount of 0.3 to 10 % by weight, preferably 0.5 to 8 % by weight, more preferably 0.8 to 6 % by weight, still more preferably 1 to 5 % by weight, magnesium is contained in an amount of 5 to 35 % by weight, preferably 8 to 30 % by weight, more preferably 10 to 28 % by weight, still more preferably 12 to 25 % by weight, and halogen is contained in an amount of 30 to 75 % by weight, preferably 35 to 75 % by weight, more preferably 38 to 72 % by weight, still more preferably 40 to 70 % by weight. It is preferable that the solid titanium catalyst component of the invention further contains an electron donor in addition to the essential ingredients of titanium, magnesium and halogen. In this case, the electron donor is desirably contained in an amount of 0.5 to 30 % by weight, preferably 1 to 27 % by weight, more preferably 3 to 25 % by weight, still more preferably 5 to 23 % by weight. The electron donor is, for example, the later-described electron donor (a). Above all, a diether compound having a fluorene ring, that is represented by the formula (i) , is preferable. The composition of the solid titanium catalyst component is determined in the following manner. The solid titanium catalyst component is sufficiently washed with a large amount of hexane and dried for not shorter than 2 hours under the conditions of 0.1 to I Torr and room temperature. Then, the solid component is measured by means of ICP (atomic absorption spectrometry), GC (gas chromatography) or the like. In the solid titanium catalyst component of the invention, the microcrystalline size of magnesium halide (microcystals) constititing the catalyst component, as calculated from peak of the (110) face measured by X-ray diffractometry of the magnesium halide, is in the range of 3 to 100 Å, preferably 5 to 80 Å, more preferably 10 to 40 Å, still more preferably 10 to 30 Å. When the microcrystalline size is much smaller than 3 Å, the particle shape of the catalyst becomes worse and the apparent bulk density of the resulting olefin (co)polymer is sometimes lowered. When the microcrystalline size is much larger than 100 Å, decrease of polymerization activity or lowering of stereoregularity of the resulting olefin (co)polymer may occur. In the solid titanium catalyst component of the invention, the volume of pores having a radius of not more than 0.1 μm is not more than 0.20 cm3/g, preferably not more than 0.15 cm3/g, more preferably not more than 0.01 cm3/g, still more preferably not more than 0.005 cm3/g; and the volume of pores having a radius of 0.1 to 7.5 μm is not less than 0.30 cm3/g, preferably not less than 0.40 cm3/g, more preferably not less than 0.45 cm3/g, still more preferably not less than 0.50 cm3/g. When the volume of pores having a radius of not more than 0.1μm is much larger than 0.20 cm3/g, decrease of polymerization activity or lowering of stereoregularity of the resulting olefin (co)polymer may occur. When the volume of pores having a radius of 0.1 to 7.5 μm is much smaller than 0.30 cm3/g, decrease of polymerization activity may occur. The mean particle diameter (volume standard) of the solid titanium catalyst component of the invention, as measured by a light transmission sedimentation method, is in the range of 0.5 to 80 μm, preferably 3 to 70 μm, more preferably 3 to 35 μm. When the mean catalyst particle diameter is much smaller than 0.5 μm, the resulting olefin (co)polymer sometimes contains a fine powder. The microcrystalline size, the volume of pores and the mean catalyst particle diameter are measured in the following ways. Microcrystalline size The microcrystalline size was determined by measuring a half-value width (FWHM) of the (110) face by means of an X-ray diffractometer (RU-300 manufactured by Rigaku Denki Co.) and applying the obtained value to the known Scherrer's formula (0.9 in the formula returns to a constant K). The samples used in the measurement of the microcrystalline size were all handled in a nitrogen atmosphere. The measurement of the microcrystalline size using the Scherrer's formula is described in detail in "Elements of X-ray Diffractometry by D.B. Cullity" (translated by Gentaro Matsumura) published by Agne Co. Volume of pores About 0.3 g of a sample for measurig pore volume was accurately weighed and introduced into a measuring cell. After the cell was degassed (to about 0.7 Pa), mercury was poured into the cell, and the cell was mounted on a device to measure the pore volume. The measuring conditions are as follows. Measuring device: Porosimeter 2000, manufactured by Carloelva Co. Measuring pressure range: about 1,000 kPa to. 190 MPa Measuring mode: pressurizing process within the above pressure range Cell volume: 15 cm3 Mean catalyst particle diameter (volume standard) Measurement of the mean catalyst particle diameter was carried out by a light transmission sedimentation method. In the measurement, an automatic particle distribution measuring device of CAPA-300 model (manufactured by Horiba Co.) was used, and the main particle deameter was calculated by applying the obtained values to the known Stokes' formula as shown below. A mixed liquid of decalin and triolein (decalin/triolein = 4/1 by weight) was used as a dispersant. (Formula Removed) D: particle diameter (cm) ηo: viscosity coefficient of dispersant (p) p: density of sample (g/cm3) p0: density of dispersant (g/cm3) t: sedimentation time (sec) X1: distance from the center of rotation to the sedimentation surface X2: distance from the center of rotation to the measuring surface ω: angular velocity (rad/sec) (Preparation of solid titanium catalyst component) There is no specific limitation on the process for preparing the solid titanium catalyst component for olefin polymerization according to the invention. For example, the following processes are available. (1) A magnesium compound in the liquid state is contacted with a liquid titanium compound in the presence of an electron donor (a). (2) A magnesium compound in the liquid state is contacted with an electron donor (a). Then the resulting solution is contacted with a liquid titanium compound (titanium compound in a liquid state) and, optionally, is further contacted with an electron donor (b) and a liquid titanium compound. Next, the materials used for preparing the solid titanium catalyst component are described. Magnesium compound in the liquid state The magnesium compound in the liquid state can be prepared from magnesium compounds having reducing ability or those having no reducing ability. The magnesium compounds having reducing ability referred to above include, for example, organomagnesium compounds represented by the formula XnMgR2-n wherein n is 0 m > 4, and n is a number of 0 < n < 4; 2. The process as claimed in claim 1, wherein the magnesium compound, in . the liquid state i's prepared by contacting a halpgen-containing'magnesium compound with a compound capable of solubilizing the halogen-containing magnesium compound and selected from, the group consisting of alcohols, esters and ethers in a hydrocarbon solvent. 3 A process for preparing a solid titanium catalyst component substantially as herein described with reference to the foregoing examples.