A rigorous electromagnetic theory was developed to compute the speckle of a 2D random rough surface with perfect conductivity, when the incident wavelength is of the same order of magnitude as the lateral size of the asperities (resonance domain). The failure of Beckmann's theory in this domain is shown. On the other hand, a new approximate theory leads to accurate results when the mean depth of asperities is small with respect to the wavelength. A comparison of numerical results with some predictions of speckle theory is made. In particular, the limit between partially developed speckle and developed speckle is studied both numerically and theoretically.