The Spex mixer mills are widely used in the industry to synthesize a variety of materials. In the present study, the discrete element method (DEM) simulation technique is used to understand the motion of the grinding media and the effect of the size of the grinding media and the mill length on grinding. The simulations correspond to a grinding chamber with grinding media only. The total mass of grinding media in different numerical experiments with different grinding media sizes is kept constant. The average collision intensity per ball, which is defined as the product of the median of the contact force distribution and the number of collisions normalized to the number of grinding media, is used as the parameter to study the quantitative differences. The collision intensity distribution shows an Optimum as a function of the grinding media diameter. Moreover, it was found that the collision intensity varies with the length of the grinding chamber.