Light scattering in the backscattering geometry, and transmission measurements are used to size monodisperse spherical particles in solutions. The concentration of spheres spans the scattering to range from single scattering to very highly multiple scattering. These experiments were performed by varying volume fractions of polystyrene latex spheres of nominal diameter 0.136 and 1.015 m. Transmission measurements were performed as a function of particle size, volume fraction, and sample thickness with a spectrophotometer equipped with an integrating sphere. The data show that for thin samples the percent transmission is nearly independent of particle size and depends only on L/l*, the ratio of the sample thickness L to the transport mean free path l*. By fitting the autocorrelation functions of samples obtained from known sphere sizes to a generalized cumulants model, which describes both the highly multiple scattering regime and the singly scattering regime, parameters were found that allow particle sizes to be determined over a wide range of scattering strengths. For samples of unknown size, this method can size monodisperse spherical particles in highly scattering solutions to better than 10%.