<p>The mechanical behaviors of rock mass containing discrete fracture networks (DFNs) are always a challenging topic in rock engineering. A methodology is presented that incorporates the generation of synthetic rock mass (SRM) and investigates its mechanical behavior using a GPU-based discrete element method (DEM) software, CoSim-DEM. This approach considers factors such as fracture intensity, variations in domain size, and other relevant parameters. The particle size ratio (PSR), which means the ratio of the particle size to the characteristic scale of the sample, is used to analyze the influence of the element/particle size on DEM numerical results. When the PSR is less than 0.1, it has little influence on the numerical results for both rock and rock mass samples. The numerical tests indicate that both fracture intensity and sample size significantly affect the failure and mechanical behaviors of rock mass samples. Based on a generated SRM, rock mass samples with different sizes are generated and used for numerical uniaxial tests. The results indicate that the rock mass exhibits an obvious scale-dependent characteristic, with the representative elementary volume (REV) being approximately 1.5 times the maximum size of the fractures. This study may provide a novel approach to the study of rock mass mechanics and the development of numerical test methods.</p>