The motion of solute molecules in the crowded milieu of biological cells as revealed by fluorescence correlation spectroscopy (FCS) can appear to be anomalous, implying that the solute mean squared displacement (msd) does not vary linearly with time. We have used a Brownian dynamics/Monte Carlo simulation method to investigate the effect of diffuser size and crowder volume fraction on msd and FCS curve shapes. For soft repulsive crowder spheres of 150 nm radius and soft repulsive diffuser spheres of 0.7 nm radius, msd plots of the diffuser remained linear up to a crowder exclusion volume of 63%. Corresponding FCS curves were well described by a simple diffusion model. In contrast, msd plots of diffuser spheres of 150 nm radius in the same crowder were clearly nonlinear at 25% volume exclusion. Corresponding FCS curves were described fairly well by an anomalous diffusion equation derived from a model in which the msd varied as t^α, where α<1. These results indicate that a key parameter describing crowding is the size of the diffuser relative to the crowder. These results also suggest that a good description of FCS curve shape can be obtained from a simulation of msd which requires a factor of 100 less simulation time.