A plane strain analysis of a deformation and stress field in cohesionless granular bodies during shearing in a direct shear tester was performed with a finite element method on the basis of a hypoplastic constitutive law enhanced by polar quantities: rotations, curvatures, couple stresses, and a mean grain diameter used as characteristic length. The constitutive law takes into account the effect of pressure, void ratio, direction of deformation rate, mean grain diameter, and grain roughness on the material behavior. The FE calculations were carried out with a different initial void ratio, vertical load, mean grain diameter, and specimen length. Attention was focused on the size effect caused by the size of microstructure related to the specimen dimensions and the effect of side boundaries on the shear zone formation. The FE results show that the thickness of the shear zone increases with increasing initial void ratio, pressure level, mean grain diameter, and specimen length. Due to the effect of boundary conditions, the thickness changes along a horizontal midsection (it is widest in the mid-region).