A realistic size of 6" would probably be fair as a maximum.
Peaceful: Angelfish would be fine, so would Festivums which are not likely to get to their maximum quoted. Discus unlikely as best in groups, but not impossible in a smaller group. Keyholes would have more than enough room.
More Aggressive: Blue Acaras are quoted up to 8" but I would say that is exceptional so they would probably be ok. Bandit cichlids are like a more colourful and interesting version of the Keyhole, they would be good.
A few examples there.
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