This paper studies the effect of model scale on the simulated mechanical properties based on PFC3D modeling. The uniaxial compression tests of intact rocks are simulated. Three different particle size ratios, all having a uniform particle size distribution and the same average diameter, are considered. For all simulations, the same model length to diameter ratio of 2 and the same microscopic mechanical properties are utilized. By changing the model size represented by the model length to the average particle diameter ratio (L/d) at each particle size ratio, the unconfined compressive strength (UCS), Young's elastic modulus (E), and Poisson's ratio are determined based on the PFC3D simulations. The results show that the coefficients of variation of the simulated mechanical properties in PFC3D decrease significantly as L/d increases. To reach a specific variation level for the simulation results, the related minimum model size should be used. The results also show that both UCS and E increase with larger L/d, although the rate of increase decreases with L/d. The analysis indicates that the increase is caused by the decrease of model porosity with larger L/d. As for the Poisson's ratio, the scale effect can be ignored when L/d is large enough.