Using transmission electron microscopy, the size-dependent room temperature oxidation of tin nanoparticles is studied. The oxide that forms during room temperature oxidation of Sn particles is amorphous SnO, and it retains this stoichiometry and structure over extended time periods. From the investigation of arrays of Sn nanoparticles with broad size distribution, under identical conditions, the Sn oxide thickness is evaluated as a function of size and oxidation time. The oxide thickness depends strongly on the size of the Sn nanoparticles, which is in excellent agreement with predictions for a Mott-Cabrera model corrected for a non-uniform electric field. The results demonstrate the accelerated oxidation kinetics of nanoscale particles with high curvature, due to the amplified electric field at the interface to a continuously shrinking metal core.