Pulsed field gradient nuclear magnetic resonance (NMR) is used to estimate the size of quantum dots (QDs) produced in a simple, fast, small-scale synthesis. This upper-division laboratory experiment teaches the basics of QD synthesis and pulsed field gradient NMR, a powerful technique for measuring diffusion coefficients in solution. The diffusion coefficients are used to calculate hydrodynamic radii, which are compared to core radii obtained from optical absorbance sizing curves, providing insight into the structure of QDs in colloidal suspension and the nature of the information provided by pulsed field gradient NMR.