Optical trapping of single nanoparticles in vacuum has various applications in both precise measurements and fundamental physics. However, to date, the number and size of randomly loaded nanoparticles in an optical trap is difficult to determine unless in vacuum. In this Letter, an efficient method for nanoparticle size estimation in an optical tweezer system before the evacuation of air was proposed and demonstrated experimentally, using scattering light from levitated particles. The particle radii deduced from the scattering light power in our proposal and from the kinetic theory of particles in gas match well (with the differences of less than 10%). For sample particles with radii ranging within 50-100 nm, we also provide a preselection rule based on this method, where over half of the trapped particles are verified as single particles. Such a particle analysis method is applicable also for the size estimation of levitated diamond particles, gold particles, and other plasmonic particles and can be applied to discovering novel scattering effects.