The adult male King Eider has a bright orange bill and shield and a dark back. Immature male Common Eider has a dark bill while the immature male King Eider has an orange bill. Female King and Common Eiders are very similar but note the barring on the flanks; linear in Common, arrow-shaped in King. King Eiders have steeper foreheads than Commons and the feathering on the bill is different-in the Common Eider it extends farther from the side of the bill toward the nostril and less far down the forehead than the King.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.