About 6" but they need room because they are, after all, cichlids and can be quite nasty once they start breeding/pairing up. They are also very 'tall' so they need a tank that's at least 16-18" in height, making them unsuitable for many standard-shaped aquariums which would otherwise hold enough volume. If you do get angels, keep in mind that they can and will eat small fish (like neon tetras) and that their long fins also mean they can be targeted by nippers (such as tiger barbs). A 30 gallon (tall is prefferable over long in this case) is probably a good size for a pair if you want a community set-up.