How big do Oscars really get?

Oscars really can get over 13", my friend has two in her 100 gallon tank, they are bigger than dinner plates! But in all truth it is rare, fish will only grow to the size the tank allows, and they have to be kept well maintained, her fish are probably very old. No one is exaggerating, but they probably have only seen the fish label at the pet store saying they CAN get that big, most don't.