This chapter presents a discrete element method (DEM) study on the scaling effect of strain localization in dense sand. Disc particles and clumped particles were used to generate samples for biaxial compression tests, and a flexible boundary was adopted. To investigate the scaling effect, specimens of fixed size composed of particles of different sizes were employed. Numerical simulations reveal that the stress difference and volumetric strain at a given axial strain decrease with increasing R (the ratio of the specimen width to the mean particle size d50). When R exceeds 30, the stress difference, volumetric strain, peak friction angle, and maximum dilatancy angle converge to stable values. The influence of R on the shear strength and volumetric strain is more significant in samples composed of disc particles than in those made of clumped particles. As R increases, the coordination number and the average and maximum contact forces gradually decrease, and the shear band width in dense sand decreases and converges to a constant value. The shear band inclination does not significantly vary with R, and the results obtained are consistent with Roscoe’s formula. When R is sufficiently large (i.e., R ≥ 40), the shear band width becomes constant, and the influence of the particle size on the shear band vanishes.