This paper investigates the influence of the length scale of a cohesionless granular soil specimen on the evolution of shear resistance in the direct shear interface test. Numerical simulations are carried out for two different specimen sizes using a micro-polar hypoplastic material model and the finite element method in the updated Lagrange frame. Due to the presence of the lateral rigid boundaries of shear box, the deformation and stress fields become significantly inhomogeneous. Correspondingly, the mobilized shear resistance and the shear band thickness are not constant along the interface. It is shown that, for the case of medium rough wall, some shear localization occurs along part of the interface between the sand specimen and the rough bottom surface and some shear localization takes place within the sand specimen. The evolution of the average mobilized friction angle obtained from the interface shear test is also influenced by the scaling effect of the testing device particularly at the beginning of shearing.