(copyright) 2022 Elsevier B.V.We report the effects of radius-, length- and pitch-sizes on the optical reflectance of a periodic square array of GaAs nanowires embedded in epoxy. The simulated system is a multilayer array constituted by alternating layers of epoxy and an effective medium of GaAs nanowires embedded in epoxy. For both s- and p-polarizations, we observe an oscillating behavior in the reflectance spectra, as a consequence of interferences in periodical systems. We found that the radius- and pitch-sizes significantly affect the reflectance of GaAs nanowires array, while the length-sizes do not present evidence of changes in the optical reflectance. For higher radius, the number of oscillations increases and consequently, the peak-to-peak distance decreases. Besides, there is a red-shift of the reflectance for increasing radius. For higher pitch, the number of oscillations also increases, and a red-shift is observed. We obtain dependence laws for the peak-to-peak distance and red-shift versus radius and versus pitch. These dependences obey approximate quadratic relations. Attending to the reflectance dependence on the light incidence angle, we have found that for s-polarized light, the reflectance is higher with increasing angles, in comparison to p-polarized light cases, independently of the radius and pitch values. For both polarizations, we found that the reflectance is increasing for greater radii and smaller pitches, independently of the incident angle.