How can I tell the difference between a little brown bat and a big brown bat?

It can be quite difficult for the non-professional to properly identify a specific bat. Nevertheless, here are a few guidelines to help you distinguish between little and big brown bats.

Little Brown Bat (Myotis lucifugus)
Forearm — 1.34 to 1.61 inches (3.4 to 4.1 cm)
Wingspan — 9.02 to 10.59 inches (22.9 to 26.9 cm)
Ears — 0.55 to 0.63 inches (1.4 to 1.6 cm)
Foot — approximately 0.39 inches (1.0 cm); long hairs on toes extend beyond claws.
Color — Pale tan through reddish brown to dark brown, depending on geographic location. The species is a rich dark brown in the eastern United States and most of the West Coast. Fur is glossy and sleek.

Confusion may occur with a few other “house” bat species. In the East, it may be confused with Keen’s bat (M. keenii), which has longer ears [0.69 to 0.75 inches (1.7 to 1.9 cm)] and a longer, more pointed tragus (the appendage at the base of the ear). In the West, it resembles the Yuma myotis (M. yumanensis), which has dull fur and is usually smaller. However, the Yuma myotis and little brown bat may be indistinguishable in some parts of the northwestern United States where they may hybridize.

Big Brown Bat (Eptesicus fuscus)
Forearm — 1.65 to 2.01 inches (4.2 to 5.1 cm)
Wingspan — 12.80 to 13.78 inches (32.5 to 35.0 cm)
Ears — with rounded tragus
Color — From reddish brown, copper colored, to a dark brown depending on geographic location. This is a large bat without distinctive markings.

Confusion may occur with the evening bat (Nycticeius humeralis), though the latter is much smaller.

Resources: (online)