Body size variation among animals has many possible correlates, temporal as well as geographic. Adult male body size was analysed over the course of 23 yr (1989–2011) in a population of Fowler’s toads Anaxyrus fowleri at Long Point, Ontario. We used an information theoretic approach to identify the most likely models to explain body length variation in relation to abundance, age and environmental variables, including temperature. Male toads overall averaged 53.6 ± 0.1 (SE) mm (n = 1976) but average body length from year to year varied from 50.9 ± 0.2 to 61.4 ± 1.3 mm (n = 23 yr), a difference of 18.7%. Abundance was the only variable significantly correlated with body size variation (R²= 0.713, p = < 0.001). A significant 10-yr trend in increased body size (R²= 0.874, p = < 0.001) was coincident with a previously detected negative trend in abundance. A 0.05°C yr−1 increase in environmental temperature over the course of our study was not significantly correlated with the toads’ body size. Body size variation in these toads is likely related to density-dependent resource availability for growth in the terrestrial stage. Temporal changes in average body size within populations in relation to density may be a significant component of phenotypic variation.