The relatively nondescript Eastern Wood-Pewee is almost identical to the Western Wood-Pewee and is best separated by differences in range and songs. The larger Greater Pewee has an entirely yellow lower mandible. Olive-sided Flycatcher is quite similar but is larger, with more obvious dark sides, larger bill, and sometimes has white patches visible above the wing. Empidonax flycatchers are very similar but appear smaller, have more obvious eye rings (most species), and different vocalizations. Juvenile Eastern Phoebe is similar but has a darker head and wags its tail.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.