Least Bittern is much smaller with buff wing coverts. Immature Night-Herons have white spotting or streaking on the upperparts, lack the black malar streak, have thicker bills, and lack the black primaries and secondaries (a useful field mark in flight).
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.