We characterize slurries composed of ceria nanoparticles in the range of 100-400 nm by exploiting a new approach which makes the data interpretation independent of any a priori assumption about the sample and provides very accurate and precise measurement of the particle size distribution, irrespectively of the huge polydispersity. The complex field scattered by single particles is determined by simultaneous measurements of the extinction cross section and the forward-scattered field amplitude. Moreover, we show how this approach overcomes typical issues encountered with this kind of suspensions such as the presence of aggregates, spurious components, and gas bubbles, at variance with any other method for measuring single particles. Applications are discussed.