Particle dynamic simulations were carried out to examine the effect of microstructure formation on the stress tensor in a sheared granular material. Clusters, which form due to the inelastic nature of collisions between the particles, have been observed by previous researchers in both experiments and computer simulations. By varying the parameters of the system, the effect of clustering has been examined for simply sheared systems of inelastic disks and spheres. It was found that the long-time average stresses and granular temperature initially increased with the system size and then approached limiting values as the size of the system was further increased. It was also observed that the distribution of stresses initially broadened with system size and then approached limiting distributions as the size of the system was further increased. Combining the values of the stresses obtained from our simulations with a previous theory for the clustering length scale, it has been determined that once clusters are fully formed in a system, the stresses no longer increase. These results suggest that under certain circumstances a small system can capture the overall behavior of a much larger system, and by knowing the variation of stress with system size it is possible to determine whether or not the effect of clustering is significant for a particular application. In addition, we conclude that it is important to identify relevant microstructures in granular flows and simulate a large enough system so that the effects related to these microstructures are not missed.