Monte Carlo simulations of adsorption and reaction of polyatomic molecules have been conducted on regular Euclidean shapes and on random tessellations to determine the extent to which simple geometric parameters, such as particle size and the spatial distribution of active sites, determine the selectivity and activity characteristics of supported-metal catalysts. It is observed that the influence of low coordination metal atoms at the edges and corners of metallic clusters become significant at an average particle size of approximately 5-10 nm. This is in agreement with experimental results for a range of hydrocarbon reactions catalysed by supported metals. The influence of molecule size and geometry has also been considered in these simulations, and it is demonstrated that the influence of molecule and particle geometry on the particle effectiveness factor is insignificant. By investigation of various combinations of active site distribution and particle size it has been possible to reproduce the main features observed from experimental studies relating metal particle size and catalyst performance, by consideration of geometric parameters alone.