Body size is a key ecological trait influencing trophic relationships, geographic distributions, and many other aspects of animal interactions with the environment. For poikilotherms, body size has a strong functional relationship to thermoregulation. This relationship means that body size evolution in reptiles may be related to climatic change over time. In this thesis, I describe histories of maximum body size change in reptiles across the Cenozoic and examine which factors drive reptile size evolution at different scales. I characterize maximum body size trends of crocodylians, turtles, lizards, snakes, and birds across all terrestrial continents through the Cenozoic. My results show coordinated trends across groups, most notably with size peaking in the Late Miocene in all groups except for squamates. This finding refutes the claim that, following the extinction of the dinosaurs, terrestrial mammals were unique in experiencing an ecological release allowing them to fill large-bodied niches. Trends also show association between size increase and global cooling. To test whether this pattern observed over geologic time holds across space in the modern world, I model the relationship between climate and body size distributions in modern turtle communities. There are significant relationships between temperature and turtle body size, with larger size associated with higher temperatures, which support predictions from metabolic theory. Conflicting relationships between size and temperature observed in the past and present suggest that, across the Cenozoic, the influence of temperature was superseded by other factors facilitating size increase. Such factors potentially include interactions between animal groups, as supported by correlated size histories, and changes in habitat availability as Cenozoic cooling increased environmental heterogeneity. As a case study tracing the effects of changes in habitats on reptile size at smaller scales, I examine reptile body sizes at sites from the Plio-Pleistocene of East Africa. Reptile body size reconstructions from the Shungura Formation (3.6-1 Ma) of southern Ethiopia indicate that size evolution is linked to local-scale changes in the environment, such as vegetation structure and lake level. These findings inform understanding of what factors limit or enable the evolution of giant reptiles and, more broadly, how local and short-term environmental influences shape trait change in a way that scales up to deep-time trends.