Most other large white birds lack black flight feathers. White Pelican much different in shape, tucks its neck in while flying, has short legs and a long, orange bill. Whooping Crane has white secondaries and a white, feathered head and neck. White Ibis is much smaller with white head and neck, reddish bill and white secondaries.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.