Often experimentalists study particulate samples that are nominally monodisperse. In reality, many samples have a polydispersity of 4-10%. At the level of an individual particle, the consequences of this polydispersity are unknown as it is difficult to measure an individual particle size from images of a dense sample. Here we propose a method to estimate individual particle radii from three-dimensional data of the particle positions. We first validate our method with simulations. We then apply our method to experimental data of colloidal suspensions observed with confocal microscopy. We demonstrate that we can recover the full particle size distribution in situ. Finally, we use our method to study the relationship between homogeneous colloidal crystal nucleation and particle sizes. We show that nucleation occurs in regions that are more monodisperse than average.