Numerical simulations of rough surface scattering at near-grazing incidence require very large surfaces (≳500λ). Conventional methods of exact solutions require the inversion of a very large matrix, which can exceed the memory and speed capabilities of even modern supercomputers. The beam simulation method proposed by Saillard and Maystre circumvents this problem by decomposing the large incident beam into narrower subbeams and then synthesizing the large beam by coherent superposition. The radius of these narrower subbeams is determined by the local interaction distance on the surface, which is found to increase with incidence angle, ultimately forcing a single beam in the limit of strict grazing incidence. This paper demonstrates that this technique gives essentially the same results as can be obtained by the method of moments and can handle surfaces as large as 1000λ for grazing incidence angle as low as 10°.