Metabolic theory predicts that maximum body size should correlate with environmental temperature in reptiles. But studies of this relationship in extant reptiles have not revealed consistent patterns among higher order taxonomic groups. Could there be a relationship between these variables over millions of years? Here, I test the hypothesis that body size range in assemblages of lizards and crocodylians correlates with transitions in environmental temperatures over geologic time intervals. I estimated snout-vent length (SVL) using regressions from extant specimens for 274 lizard and 234 crocodylian specimens from the Western Interior of North America through the Paleogene (66-23 Mya), which spans several warming and cooling events evidenced by terrestrial and marine proxies. My results indicate that maximum lizard SVL has a positive linear relationship with local terrestrial temperature across this geotemporal system. These results reflect some recent findings that local temperature may correlate with body mass in extant lizards. In contrast, crocodylian SVL does not track local terrestrial temperatures across this system, but does indicate a positive linear relationship with concurrent global marine temperatures. This result could reflect the aquatic habits of crocodylians, or possibly migration into the interior basins from lower latitudes. Other studies of this relationship in fossil and extant crocodylians have yielded contradictory results and suggest that body size in this group may be more associated with ecology and resource availability than with ambient temperature. While the relationship in crocodylians remains elusive, this study offers new evidence to suggest that maximum lizard body size may be constrained by terrestrial environmental temperature on both ecological and evolutionary time scales.