The validity of Bergmann's rule, perhaps the best known ecogeographical rule, has been questioned for ectothermic species. Here, we explore the interspecific version of the rule documenting body size gradients for anurans across the whole New World and evaluating which environmental variables best explain the observed patterns. We assembled a dataset of body sizes for 2761 anuran species of the Western Hemisphere and conducted assemblage-based and cross-species analyses that consider the spatial and phylogenetic structure in the data. In accordance with heat and water-related explanations for body size clines, we found a consistent association of median body size and potential evapotranspiration across the New World. A relevant role of water availability also emerges, suggesting the joint importance of body size for thermoregulation and hydroregulation in anurans. Anurans do not follow a simple Bergmannian pattern of increasing size towards high latitudes. Consistent with previous regional findings, our Hemisphere-wide analyses detect that the geographic variation in anuran body sizes is highly dependent on a trade-off between heat and water balance. The observed size-climate relationships possibly emerge from the interplay between thermoregulatory abilities and the benefits inherent to reduced surface-to-volume ratios in larger species, which decrease the rates of evaporative water loss and favour heat retention. Our results also show how temperature becomes important for species that are directly in contact with the substrate and water, like burrowing and terrestrial anurans, while arboreal species exhibit a body size cline linked with potential evapotranspiration.