Sinipta dalmani Stal (Orthoptera: Acrididae) is an Argentinean grasshopper that shows considerable variation in adult male body size in natural populations at “El Palmar” National Park (Entre Rios, Argentina). Part of this variation is due to the M4 pericentric inversion polymorphism. A study of male mating success in competition cages showed that mated males are larger and have lower inversion dosage than unmated males. In the present work, body sexual dimorphism as well as the phenotypic influence of the M4 inversion on female body size and its possible association with mating success was analyzed. A notable body size sexual dimorphism was evident, with females being significantly larger than males. The phenotypic effect of M4 on female body size reported here is in agreement with the results mentioned above for males and suggests a significant genetic (chromosome) component in the determination of third tibia length in females. Female mating success was analyzed through both univariate and multivariate approaches. The analysis of selection differentials showed that third tibia length might be negatively selected during mating. The analysis of selection gradient, demonstrated that this trait might be considered as the most probable target of selection. The female differential mating success detected in S. dalmani suggests a significant male investment in offspring.