Optical single backscattering spectroscopy can be used for sizing particles suspended in aqueous solution. In this work, we present results of backscattering spectroscopy applied to the determination of radii of calibrated spherical latex microparticles when a beam of white light is incident on the sample. From Mie calculations and Fourier analysis, we can determine the radius of particles covering the range between 0.5 and 12 µm for monomodal samples, with a mean error of 0.7 µm. To improve the accuracy, a correlation algorithm is applied that reduces the uncertainty in more than an order of magnitude and compares to the traceable error given by the manufacturer. The method can be also applied to bimodal and trimodal samples, allowing the separation of different size components.