We analyzed geographical variation in body size in males and females of nine Greek populations of the bushcricket Poecilimon thessalicus Brunner von Wattenwyl, 1891. We found significant geographic variability in P. thessalicus, with all three morphometric body-size characters (hind femur, front tibia, and pronotum) highly correlated within populations. Populations differed in mean size between three mountain ranges, and were larger on moist eastern, than on dry western slopes. We suggest that the observed smaller size of the bushcrickets on western slopes is mainly the consequence of a shorter growing season due to summer drying on western slopes. Sex-specific growth rates might contribute to the observed smaller body size in males in all populations. Rensch's rule that sexual size dimorphism (SSD) decreases as body size increases is supported, with males growing relatively larger compared to females in populations with larger body sizes. This pattern might be sexually selected, as males that produce larger nuptial gifts are favored as mates in Poecilimon bushcrickets.