It is the largest known mammal of the Cretaceous period. A few other described mammals, such as an early giant platypus, Kollikodon, may have been larger, but their fossils are only small fragments so their actual size is not known.
Repenomamus giganticus was more than 1 m (39 in) long and weighed about 12–14 kg (26–31 lb). Other species of Repenomamus, Repenomamus robustus was much smaller. It was less than 50 cm (20 in) long and weighed 4–6 kg (9–13 lb).
Unlike many other mammals of the dinosaur time, Repenomamus was not a small nocturnal insectivore. It is known that Repenomamus ate small dinosaurs, but it is not clear if Repenomamus actively hunted live dinosaurs or scavenged dead dinosaurs.