Body size. Adult mice are much smaller than adult rats. Feces. Mice feces are brown or black and about the size and shape of rice grains. Norwegian rat droppings are larger and more cylindrical, and roof rat feces are larger and more banana-shaped.Tails. Rat tails are hairless, whereas mice tails grow tiny hairs. Mouse tails are proportionately longer compared to their body size than rat tails. Ears. Mouse ears are more rounded than a rat’s, and rat ears are slightly narrower than a mouse’s.Body shape. Rats, especially the Norwegian rat, have more of a hunch to their bodies than mice.