Testudinid turtles, comprising a single family with a fairly uniform shell morphology but a wide range of body sizes, present a unique opportunity for testing hypotheses regarding the evolution of shell size, shape, and functional performance. Here, we apply geometric morphometric methods and finite element models to test two hypotheses regarding the coevolution of size, shape, and mechanical performance (shell strength). First, we predicted that larger turtles will tend to have shell shapes distinct from those of smaller turtles. Second, we predicted that the shell shapes (independent of size) of smaller turtles will be stronger than those of larger turtles to compensate for the reduction in strength that accompanies a decrease in size. We used multivariate phylogenetic comparative methods to assess the significance of the relationship between shell size and shell shape, and used finite element analysis to assess the mechanical behavior of tortoise shells. We detected a relationship between shell shape and size—the shells of larger tortoise species had larger carapaces relative to plastrons, larger vertebral and pleural scutes relative to marginal scutes, and more vertically oriented marginal scutes. We did not detect a relationship between size-independent shell strength and size, however, and the direction of the trend was the opposite of what we predicted (larger tortoises tended to possess stronger shells than smaller tortoises). This is consistent with larger tortoise species having more highly domed carapaces than smaller species. Future studies should focus on the effect that additional functional factors (such as righting ability or heat exchange), and their interactions, might have on the selective pressures that affect the evolutionary path of tortoises.