Optical trapping of liquid aerosols from polydisperse samples provides unique problems for measuring their radii. Perhaps the most precise method, cavity-enhanced Raman spectroscopy (CERS), is limited to relatively large aerosols (>2 mu m in radius). Determining particle perimeters in video microscopy lacks precision and, although simple, can be ambiguous. Here we demonstrate a simple and precise method based on studying the Brownian motion of droplets as they approach a nearby surface. We obtain results with greater precision and reliability than video microscopy, and with no size limitation conclude the technique could compete with CERS in terms of precision and accuracy.