To investigate the scale effect on the shear strength characteristics of coarse-grained soils, a numerical model of a direct shear test was developed using the PFC^3D. The micromechanical mechanism of particle size effects on both strength and deformation characteristics was analyzed. Through the examination of microstructural evolution features such as force chain distribution, displacement field, and stress field, the underlying mechanism by which particle size influences shear strength was revealed. The results indicate that both shear strength parameters cohesion and internal friction angle increase with the maximum particle size, validating the reliability of laboratory direct shear test findings. The influence of particle size on shear strength is closely related to vertical stress. Particle size significantly affects shear deformation behavior and micromechanical contact characteristics. With increasing particle size, the shear band becomes thicker and more heterogeneous. Overall, the width of the shear band is 4~8 times the average particle diameter. Additionally, force chain intensity shows greater disparity and tends to concentrate spatially. Shear deformation exhibits a progressive and cumulative development process. The principal stress orientation, fabric tensor direction, and major force chain alignment show nearly consistent deflection trends, with post-deflection directions concentrated around 30°~45°. The deflection angle also increases with particle size. These findings provide theoretical support for the numerical simulation of geotechnical tests and their practical engineering applications.