Estimating body size of extinct mammals presents problems when size can be estimated only by extrapolation. I examined the influence of phylogenetic, biomechanical, and statistical assumptions on body size estimates for 2 species of fossil castorids, the Pleistocene “giant” beaver Castoroides and the fossorial Miocene beaver Palaeocastor. Prior descriptions of Castoroides as “black-bear sized” were greatly exaggerated; new analyses estimated body masses at 60–100 kg. Estimates for Palaeocastor were similar to previously published estimates (0.8–1.2 kg). Biologically realistic size estimates were based on femur length and on interspecific data covering a wide range of body mass; skull length measurements or extrapolations from an ontogenetic single-species data set resulted in excessively large body mass estimates. Body mass must be estimated with due attention to the choice of both morphological trait and reference taxa.