The quality of laser beam phase distortions compensation as a function of the PC-SBS-mirror dimensions is inves-tigated both numerically and experimentally. The spectrum distortions character of reversed by the limited dimensions SBS-mirror beam is analyzed. It is shown, that the distortion factors acting on the reversed wave, i. e. the diffraction on the limited aperture of the mirror and SBS-mirror wave front reversing inaccuracy, influence on each other. It is found, that when dimensions of the mirror are fixed, the correction accuracy parameter decreases with the increase of the distance between the mirror and the phase distorting medium. This effect takes place when either the whole beam or its part only are intercepted by the mirror.