The effect of space charge on the performance of an Ion Mobility Spectrometry (IMS) system becomes more important as the system is made smaller. We use the SIMION software package with the Statistical Diffusion Simulation (SDS) module and SIMION's new capability to solve the Poisson equation to study the effect of space charge on ion loss and resolving power in IMS systems. We consider IMS systems ranging in length from 50 mm to 150 mm and in diameter from 8.33 mm to 50 mm with a fixed electric field of 50 V/mm. We also examine a system with a length of 50 mm, a diameter of 16.7 mm, and an electric field of 16.7 V/mm. We assume that any charge density can be injected into the IMS system, and we have obtained expressions that predict the ion loss and resolving power of IMS systems as a function of input charge density and drift tube aspect ratio (length/diameter).