Wide-ranging species typically differ morphologically across their ranges. Bergmann's rule suggests that taxa in colder environments are bigger than related taxa in warmer locations. We examined 767 painted turtles (Chrysemys picta) in ten populations near their northwestern range edge in south-central British Columbia, Canada, in conjunction with previous data, to test the hypotheses of (1) a Bergmann's latitudinal cline, and (2) that males and females show similar latitudinal variation in size. We also explicitly test the impact of high local variation on range-wide inference. Female and male turtles showed similar latitudinal clines in body size; the degree of sexual dimorphism did not change across the range. Importantly, local variation in sexual dimorphism across ponds was nearly as high as the previously observed continental variation. Indeed, we found both the lowest and the highest degrees of sexual size dimorphism that have ever been reported for this species. Further, differing criteria in the literature for identifying mature females compound the difficulty of interpreting latitudinal clines in size or dimorphism. Our results highlight the need for much more systematic local and regional sampling as inputs for latitudinal or other comparative analyses such as Rensch's rule because insufficient sampling of high local variation may mask important ecological and evolutionary patterns.