The monochromatic aberrations produced by the phase distribution reflected by resonant sub-wavelength metallic structures are studied both analytically and numerically. Even for normal incidence, the angular dependence of the re-radiated wavefront disturbs the overall performance of the reflectarray. This effect is modelled as combination of a linear and a cubic dependence. A complete numerical simulation of a multilevel focusing reflectarray is performed using computational-electromagnetic and physical-optics-propagation methods. A modified Strehl ratio is defined to show the dependence of the focused spot behavior on aperture. The irradiance distribution is dependent on the polarization state. A small-aperture focusing reflectarray has been designed, fabricated, and tested. The irradiance distribution at the focusing plane is compared with the simulated one, showing a good agreement when residual wavefront aberrations are included.