The interfacial electronic and geometric effects behind the size effect are crucial in determining the catalytic performance of supported metal catalysts, whereas they remain still unknown for most of important heterogeneous catalytic reactions such as water-gas shift reaction (WGSR). In this work, we report a profound size effect in five Pt/CeO2 catalysts among Pt single atoms, sub-nanoclusters (ca. 0.8 nm) and nanoparticles (3.8-9.3 nm) for the WGSR. The catalyst with about 3.8 nm Pt nanoparticles had the highest activity, superior to the other four Pt/CeO2 catalysts. Pt delta+-O-v-Ce3+ species served as the interfacial active sites, wherein the carboxyl pathway seemed more preferred than the formate route, and favored high activity. Such high activity and pathway preference are due to both of the electronic and geometric effects. On the one hand, the electronic metal-support interaction (EMSI) modulated by the metal deposit size tunes the activation of the CO adsorbed on Pt sites. On the other hand, the abundant interfacial oxygen vacancies for the geometric effect of providing the active sites to activate and dissociate H2O. (C) 2021 Elsevier Inc. All rights reserved.