It has been reported that textile defects can be found and classified by examining the optical diffraction pattern obtained by illuminating the defect-containing textile with a laser beam. Very little, however, can be found in the literature concerning the influence of the laser beam size on the performance of the method. We describe here a systematic method to determine an optimum beam size, and we demonstrate that the size of the illuminating beam must not be too small or too large, for the system to yield satisfactory results.