The acoustic features of vocalizations have the potential to transmit information about the size of callers. Most acoustic studies have focused on intraspecific perceptual abilities, but here, the ability of humans to use growls to assess the size of adult domestic dogs was tested. In a first experiment, the formants of growls were shifted to create playback stimuli with different formant dispersions (Deltaf), simulating different vocal tract lengths within the natural range of variation. Mean fundamental frequency (F0) was left unchanged and treated as a covariate. In a second experiment, F0 was resynthesized and Deltaf was left unchanged. In both experiments Deltaf and F0 influenced how participants rated the size of stimuli. Lower formant and fundamental frequencies were rated as belonging to larger dogs. Crucially, when F0 was manipulated and Deltaf was natural, ratings were strongly correlated with the actual weight of the dogs, while when Deltaf was varied and F0 was natural, ratings were not related to the actual weight. Taken together, this suggests that participants relied more heavily on Deltaf, in accordance with the fact that formants are better predictors of body size than F0.