A simple question that doesn't have a simple answer. The standard mesurment is from the nose to the caudal peduncle (body lenth). Some Angelfish can get pertty big but it usually takes a few years to reach full size. The first year they grow pretty fast but the growth slows after that. The type of angelfish also has a lot to do with max potential. Wild angelfish species range in size from the tallest P. Altum, P. Scalare and P. Leopoldi being the smallest species. Domestics are another story with some veils getting to 15" from tip of dorsal to tip of anal fin. Body size is the only fare way to measure when comparing spices because fins vary. Some domestic crosses can achieve a body size up to 6" and are a truly amazing site to behold in person. P. Altum can reach over a foot from dorsal to anal fin with a body around 4.5-5" for a full grown adult male. However tank size, water quality, diet and genetics are all factures in the ultamate size that will be attained by any particulare fish.