We estimate the activity of the oxygen reduc- tion reaction on platinum nanoparticles of sizes of practical importance. The proposed model explicitly accounts for surface irregularities and their effect on the activity of neighboring sites. The model reproduces the experimen- tally observed trends in both the specific and mass activi- ties for particle sizes in the range between 2 and 30 nm. The mass activity is calculated to be maximized for par- ticles of a diameter between 2 and 4 nm. Our study dem- onstrates how an atomic-scale description of the surface microstructure is a key component in understanding parti- cle size effects on the activity of catalytic nanoparticles.