Unmistakable at rest. Wood Stork has black flight feathers and could be mistaken at a distance, but note dark, unfeathered head and neck, which is extended in flight. Whooping Crane, Northern Gannet and Snow and Ross' Geese have black primaries and could also be confused in flight at a distance, but they all lack black secondaries.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.