In this work, we present a systematic study of the optical reflectivity of GaAs nanowire arrays as a function of nanowire size, morphology, and arrangement on the substrate. To analyze the results, we extend a recently proposed model, which describes the nanowire mat as an ensemble of diffuse optical reflectors, by including the effective medium description and the low or zero-absorption regime. The modified model is very general and well accounts for the behavior of all the samples analyzed in this work, which covers a wide range of common situations: from the array of disordered parallel oriented and small nanowires (1.5–2 μm long and 30–50 nm large) to the randomly oriented wire array with non-uniform distribution of the sizes. The results that we report here for GaAs nanowires can be applied to any other type of semiconductor nanowires and provide useful insights to design more efficient solar cells.