Magnetooptical and optical experiments are performed on size sorted maghemite ($\gamma-{\rm Fe}_2{\rm O}_3$) ferrofluids. Water based ferrofluid samples are synthesised by the coprecipitation method followed by a size sorting process. All the samples have the same particles volume fraction: 0.2%. Transmittance and Faraday rotation are found to be size dependent. The saturation of the Faraday rotation is decreasing when particles mean diameter increases. Faraday rotation curves are described by orientation Langevin theory and the size dependence of their magnitude is discussed using nanoparticles internal field.