The problem with common names they can be a bit confusing I take it its a Ameiurus nebulosus (Lesueur), Brown Bullhead they are called speckled and sometimes spotted bullheads. If it is one of these they get to 55cm about 22
Yeah, there are a quite a few bullhead species out there and they all have lots of common names. Do you have a scientific name for your fish?
All bullheads grow between 11-25" depending on the species.
I feel it important to note that bullheads all look fairly similer. You cannot ID them based on color. In fact, the color pattern on that brown bullhead is not the average for the species. Brown bullhead, on average look more like this...
Yours is probably the spotted bullhead (A. serracanthus) who exhibit a similer color pattern.