The laminated argillaceous limestone facies shale in the continental shale reservoirs of Jiyang Depression has good physical properties of reservoirs，but the mechanical properties of reservoirs have strong heterogeneity due to the dense laminae，which leads to the significant size effect of its mechanical parameters. The cores of the laminated argillaceous limestone-facies shale in Boxing Sag of Shengli Oilfield were taken as an example for systematic research on the mechanical properties of cores and the size effect of hydraulic fractures by three-dimensional finite-element numerical simulation. The results show that when the core size is less than 500 mm，the uniaxial compressive strength and modulus of elasticity have a significant size effect，and they have a negative exponential relationship with the core size under different confining pressures. The breakdown pressure of hydraulic fracturing decrease with the increase in the unit size. In Boxing Sag，when the unit size is 1.5 m，the breakdown pressure and stimulated reservoir volume（SRV）obtained by numerical calculation are closer to those of real formations. It is suggested that the unit grid with a unit size of less than 1.5 m should be used for simulation design under satisfied computational efficiency.