Within the coupled waves method complemented by quasi-optical approach, we investigate a possibility of diffraction mode selection with respect to the transverse mode index in lasers with planar Bragg resonators. Allowable values of the Fresnel parameter are determined which provide for establishment of stationary single-mode oscillation regime with narrow angle spectrum due to diffraction losses from the edges of the Bragg structure. We show that at larger values of the Fresnel parameter, oscillation stability can be improved by shifting the operating transition frequency to the eigenfrequency of the highest-Q longitudinal mode of the Bragg resonator.