In this paper a new technique for the sizing of optically levitated particles is reported. It is based upon the analysis of the dynamical behavior of the investigated particle during a transient interruption of the supporting laser beam. A mathematical model to describe the movement of the particle was developed and experiments with various samples were performed. The evaporation of single- and two-component droplets were monitored on-line. To allow a verification of the obtained results, the intensity of the light scattered by the particle was recorded simultaneously during the experiments. Comparison of measured data and computations according to Lorenz-Mie theory showed that the reported technique enables the determination of particle size with an accuracy better than 1% in less than 1 min without knowledge of its optical properties.