Semiconductor quantum dots are at the forefront of materials science chemistry with applications in biological imaging and photovoltaic technologies. We have developed a simple laboratory experiment to measure the quantum-dot size from fluorescence spectra. A major roadblock of quantum-dot based exercises is the particle synthesis and handling; the former requires dangerous chemicals while the later can lead to sample degradation. We overcome these drawbacks by using commercially available quantum dots. The samples are composed of CdSe-ZnS core-shell nanoparticles embedded in a polymer matrix. A fluorescence spectrophotometer is used to collect photoluminescence data and the subsequent calculations are straightforward. Student feedback indicates that these experiments provide a new understanding of quantum mechanics and the particle-in-a-box model. We are able to examine the role of spectroscopy in chemical investigations and a cause of spectroscopic line broadening. (Contains 1 table and 2 figures.)