The actual aquarium size is relatively unimportant. However, the number of angelfish per gallon is critical. This figure varies depending on several factors. Obviously, size has a lot to do with carrying capacity. Other variables that affect this include, pH, temperature, feeding practices, water changing volume, water changing frequency, strain of angelfish being kept and the overall quality of angelfish you desire to raise or maintain. Keep in mind, the number of angelfish per gallon a tank can handle, will vary immensely from one situation to another. The following would be a very general guideline.

Nickel size bodies  1 angelfish per gallon Quarter size bodies 1 angelfish per 2 gallons Silver dollar size bodies 1 angelfish per 3 gallons Stock ready to be paired   1 angelfish per 5 gallons Full grown breeding pair ...