Background/Question/Methods Macroecologists often focus on body size as a factor of interest because it serves as a proxy for many different aspects of an organism's biology. Size results from a complex and dynamic tradeoff between physiology, life history, environment, phylogenetic constraints, and past geologic and/or climatic history. Nonetheless, certain ‘invariant' size-dependent scaling relationships and broad-scale distributional patterns are repeatedly observed for mammals and other taxa. Do these patterns arise because of common ancestry, because organisms exist in similar environments, or because they face similar design or life history constraints? To examine these questions, we assembled a database of maximum body size for the major orders found on each continent (North and South America, Africa, Eurasia and Australia) over the evolutionary history of mammals (late Mesozoic to Recent). Maximum size was chosen because of taphonomic issues involved in characterizing minimum or mean/median size. Our temporal resolution is at the epoch or sub-epoch level (e.g., early, mid or late Miocene); data were collected from a number of primary and secondary sources and/or estimated directly from taxon-specific allometric regressions based on measurements of fossil teeth or limbs. Results/Conclusions Overall, our results clearly demonstrate a rapid increase in the body size morphospace of mammals over the Cenozoic, reflecting the evolutionary radiation that occurred after the extinction of dinosaurs and other groups at the K/T boundary. We find remarkable consistency in the maximum size achieved over time on each continent, as well as that achieved by various groups of the same guild, although geologic factors influence the distribution of orders among the continents. We also see interesting interchanges over time with some groups replaced by ecologically similar but phylogenetically distinct orders that appear to occupy the same body size niche. Our results indicate the presence of “body size niches” that are filled repeatably over evolutionary time.