Browns get larger than blacks. I've personally caught browns over 20" from a pond on my grandparents' property, and I've seen photos of bigger ones.Personal experience from pulling black bullheads out of my old fishing hole prompted me to do some research because in my life I'd never pulled one out bigger than 12" long so telling me that they get at least 24" I felt slighted. According to the dept of Natural Resources, average adult size of an Ameiurus nebulosus is 8-14".