Faraday rotation measurements have been made in samples composed of γ-Fe2O3 nanoparticles dispersed in a SiO2 matrix. All the samples show the same Fe/Si molar ratio but different diameters of the magnetic nanoparticles. A magneto-optical method to measure the particle size using the Faraday effect is presented. The increase of Faraday rotation with the mean diameter of the γ-Fe2O3 particles can be associated with surface effects. The thickness value of their external layer with disordered spins has been obtained using a simple model and its high value suggests the large influence of the matrix on the magnetic properties of the composite.