The dependence of magnetorheological properties on particle size is studied, using a model magnetorheological fluid consisting of nonmagnetic silica particles suspended in a ferrofluid. For small particles a strong increase of magnetorheological properties with particle size was found. At a certain particle size and field strength a crossover occurs to a regime where there is only a very limited size dependence. This can be understood in terms of the length of the chains in relation to the gap size of the geometry. Crossover occurs when the average chain length becomes comparable to the gap size. The remaining size dependence may be explained by polydispersity, variations in the morphology, variations in the sterically stabilizing layer of the particles and surface roughness of the geometry.