in the wild on average they grow to about 14" within 4 years, but I understand the reluctance
I kept a bullhead when I was a kid, as muddy and ugly as these fish are when caught wild, they look GREAT when raised in an aquarium in cleaner water. They get a very dark almost blueish tinge, and look really clean, so if you were able to catch some as small as like 6", i think they'd make a great fish for your tank