A series of Pt/Al2O3 catalysts with uniform Pt particle sizes ranging from 0.95 to 2.62 nm was synthesized using the refilling method to control the growing of Pt nanoparticles and evaluated in propane dehydrogenation. An effect of particle size on the initial propane activity and TOF was demonstrated, the smallest particles being the most active. A geometric model was developed to quantify the surface site concentration for any fcc metal particle size. Comparing the evolution of initial activity for propane dehydrogenation with the different surface site concentrations, it was shown that the active sites of Pt/Al2O3 catalysts corresponded to the combination of corner and edge atoms. To get further, the model was confronted with numerous data extracted from literature demonstrating that it is an efficient tool to predict the structure-activity relationship of fcc metal catalysts and determine the exact nature of active sites for a wide range of reactions and metals.