To apply the knowledge of reaction mechanisms of heterogeneously catalyzed reactions on the atomic scale to supported catalyst systems, a detailed description of the structure of active particles on the atomic scale is required. In this article, a method is developed to construct atomic-scale geometric models for supported active fee metal nanoparticles, based on a measurement of particle sizes and particle volumes by Scanning Transmission Electron Microscopy (STEM) and the M-M coordination number determined from EXAFS. The method is applied to supported Au/TiO2, Au/MgAl2O4-, and Au/Al2O3 catalysts. These geometric models allow for estimation of geometric properties, such as specific Au surface area, metal-support contact perimeter, metal-Support contact surface area, edge length, and number of Au atoms located at the corners of the particles, with an error on the order of 20%. In the three catalysts studied here we find that the Au particles in the Al2O3 supported catalyst are small. The Au particles in the Au/TiO2 catalyst are smaller in diameter than those for the Au/MgAl2O4, but also thicker. The differences in particle size and shape seem to reflect the differences in the metal-support interface energy in the three catalyst systems.