We present a method which uses the behavior of the scattered light intensity as a function of the scattering wave vector in the Guinier to power law crossover regime to determine the width of the size distribution of fractal cluster aggregates. A graphical analysis is presented and data for soot aggregates is used to illustrate the method. This analysis relies upon knowledge of the fractal cluster dimension and correlation function cutoff, and we discuss how uncertainties in these parameters lead to uncertainties in the size distribution width.