The very similar adult Nazca Booby has an orange, not yellow, bill. Adult Masked Booby can be separated from other black and white boobies by the entirely dark secondaries and dark tail. Immature boobies are very difficult to identify. Brown and Red-footed have darker bellies. Blue-footed has a white rump patch and a more horizontal demarcation between the dark upper breast and the white belly. Northern Gannets tend to lack the white collar but at a distance may be inseparable. Immature Nazca is very similar to immature Masked but may have more white in the central rectricies, lack or have an ill-defined light collar, appear to have a distinctly gray back and may acquire some adult color to the bill.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.