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Mechanisms influencing body size in freshwater turtles are complex, but important because body size influences reproductive output, survivorship, and behavior. Two proximate drivers of body size in freshwater turtles are temperature-related factors and density-dependent factors. Large museum collections from the 19th Century provide a basis by which to evaluate environmental drivers of body size. One of the largest series from the 1800s is a collection of Wood Turtles (Glyptemys insculpta) from Lancaster, Massachusetts, USA, studied by Louis Agassiz. We compared shell dimensions, growth rates, and sexual dimorphism in the Agassiz sample to a sample of living turtles from Lancaster. Adult turtles living today are 20% larger in dimensions than 1850s turtles, and 1850s males are below the modern range of variation. The sexes are more dimorphic today, and modern juveniles grow significantly faster than 1850s counterparts. We used a recent, statewide sample to evaluate whether adult body size has changed significantly as a partial response to a warmer climate, or as a density-dependent response to population decline. Modern Wood Turtle body size across Massachusetts is positively associated with growing degree-days, and adult male body size is negatively correlated with population density. Our results suggest that the 1850s growth rate may have been reduced due to a cooler growing season, and that 1850s adult body size may have been constrained by density-dependent factors. Evidence for Wood Turtle population decline at Lancaster is anecdotal, but noteworthy: Agassiz reported collecting "over one hundred in an afternoon"; we estimated approximately 2.2 turtles per survey in 2009.