Adult Masked Booby has a yellow, not orange, bill. Adult Nazca Booby can be separated from other black and white boobies by the entirely dark secondaries. 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.