Latitudinal body size-clines are primarily discussed in the context of thermoregulation, sensu Bergmann. However, body size patterns are ambiguous in ectotherms and this heterogeneity remains poorly understood. We hypothesised that the contrasting effects of thermoregulation and resource constraints obscure latitude–size relationships. Using data for 43% of all odonate species, we tested whether body size increases with decreasing temperature and increasing productivity in phylogenetically and spatially comparative analyses. We found strong but contrasting effects for temperature between Anisoptera and Zygoptera and consistent positive effects for productivity that explained 35%–57% of body size variation. We concluded that temperature, productivity, and conservatism in size-based thermoregulation synergistically determine the distribution of ectotherms, while the taxon-specific importance of these factors can lead to contrasting results and weak latitude–size relationships. Our results reinforce the importance of body size as a determinant of species distributions and responses to climate change.