The quartz thickness monitor, a high Q mechanical device, is ubiquitous in thin film deposition applications. It provides information that is one dimensional—the average thickness of the film. In this letter, we describe a method where a related device, a surface acoustic wave resonator, employed as a gravimetric sensor has been used to ascertain the radius of freestanding spherical nanoparticles. Direct measurements of particle size through scanning electron microscopy confirm the validity of this method. Its success implies that use of high Q mechanical oscillators can be extended to extract the lateral extent of nanostructures during growth.