We studied geographical body size variation in 23 populations and sexual size dimorphism in 19 populations of the grasshopper Dichroplus pratensis Bruner in Argentina, spanning 22° of latitude and between sea level and >2,474-m altitude. Six characters were measured in 609 individuals (334 males and 275 females): total body, femur 3, tibia 3, tegmina and pronotum lengths, and pronotum height. Significant negative correlations between all six characters and latitude occurred. Correlations with altitude were weaker, although individuals at higher altitudes tended to be smaller, except at high latitudes. Extreme latitudes and altitudes correspond to the geographical and ecological margins of the species distribution and share comparable strenuous environmental conditions. Thus, this species follows the converse to Bergmann’s rule, that is, individuals tend to be smaller as the environment becomes cooler (when latitude or altitude are used as a proxy for temperature). When the six morphological characters where analyzed in both sexes with respect to mean January and July temperatures at each locality, positive (although not significant) correlations were obtained in all cases. Sexual dimorphism occurred throughout the geographic range involving not only differences in body size but also in body proportions, which can be explained through differential allometric growth and shorter developmental time in males (the species exhibits protandry). The latter can also explain the inversion of Rensch’s rule that occurs in D. pratensis: sexual size dimorphism decreases as general body size increases.