When people judge the duration of stimuli, judgments are influenced by the physical size of these stimuli. Specifically, people tend to judge the duration of large stimuli longer than the duration of small stimuli. However, some authors (Bottini & Casasanto, 2010; Ma, Yang, & Zhang, 2012) have reported that even implicit size can affect duration estimates. The present study extends the results of these studies. Specifically, we examined whether the imagined size of objects would also influence duration estimates. In each trial, participants reproduced the duration of an animal word. During the presentation of the word, they were asked to imagine the word's referent. The animals employed in all experiments varied in size. In Experiment 1, participants were instructed to imagine the animals in a fixed context. Reproduced duration increased not only with the numbers of letters of the animal word (i.e., physical size) but also with the size of the word's referent (i.e., implicit size of the animal). Experiments 2 and 3 examined whether the effect of size would increase when the size of the animals was made more salient. The results of all experiments showed that reproduced duration increases with implicit stimulus size. In addition, Experiments 2 and 3 provide some evidence that the imagined size effect becomes more pronounced when the participant's attention is drawn to the size differences among the imagined animals. It is argued that the implicit size effect is mediated by mental imagery that operates on pre-existing space--time associations.