Rensch's rule describes the pattern of sexual size dimorphism, claiming that in taxa where males are the larger sex, they exhibit higher body size ratios. Domesticated animals offer excellent opportunities for testing predictions of functional explanations of Rensch's theory. In this study, we tested the hypothesis that the morphological size of sheep breeds follows Rensch's rule. We have analysed data in the literature on adult body size (live weight and withers height) of males and females in 74 sheep breeds. The analysis confirms that the pattern of sexual size dimorphism conforms discretely to Rensch's rule among sheep breeds, with all breeds appearing to be dimorphic. We propose that this is due to the fact that rams and ewes have been subjected to different selection regimes, with a higher selective pressure on rams.