Propane dehydrogenation (PDH) has shown great potential to meet the increasing global demand for propylene. However, industrial Pt- and Cr-based catalysts are either costly or toxic. The development of cost-efficient and environmentally friendly catalysts is highly desirable. Herein, we synthesized a series of SiO₂-supported ZnO catalysts (ZnO/SiO₂) from atomically dispersed species to particles of a few nanometers and to continuous films using atomic layer deposition. In the PDH reaction, we showed that the mass specific rates and propylene selectivity both exhibited a volcano relationship with the size of ZnO, where a ZnO/SiO₂ catalyst with a size of approximately 4.8 nm had the maximum PDH activity and a propylene selectivity of 95%, along with good long-term stability for at least 10 h. Photoluminescence spectroscopy and temperature-programmed desorption of NH₃ revealed that the amount of oxygen vacancies and acidity sites in ZnO had the same trend with the PDH activity as a function of ZnO size and reached the maximum on the 4.8 nm-sized ZnO/SiO₂ catalyst. This result suggests that unsaturated Zn cations accompanied by oxygen vacancies are the active sites for the activation of C–H bonds in propane.