The very large Tundra Swan is unlikely to be confused with anything but other swans. White Pelicans, Whooping Cranes, Wood Storks, and Snow Geese are all white birds that from a great distance could look like swans but all have black primaries. The adult Mute Swan can be told from the adult Tundra Swan by its orange and black knobby bill. The adult Trumpeter Swan is very similar to the adult Tundra Swan but it is slightly larger, has a straight culmen, the bill has no yellow spot, the eye is enclosed by black, and the white feathering on the head extends in a v shape into the dark bill. In the far northwest, a subspecies of the Tundra Swan shows more yellow at the base of the bill and could be confused with the Alaskan Whooper Swan. The Whooper Swan has yellow in the bill that goes beyond the nostril. We will get back to you about the immatures!
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.