I did a quick search and it appears that they grow to 18 inches in size. A large tank is definately needed.
Here's what I pulled from North American Native Fishes Association:
The yellow bullhead has been known to reach 18" in total length and to weigh well over three pounds, our fish have never exceeded 12" and are usually in the 4" - 7" range. As an aquarium fish, the yellow bullhead is a very satisfying choice for those of us who appreciate the unique characteristics of catfishes. Certainly not a beautiful fish, and one not very likely to pass up making a meal of any tankmate smaller than itself (except when very small, these fish are best housed alone), the yellow bullhead does very well in captivity. When provided with even basic care such as a tank large enough to adequately accommodate it, clean water, and plenty of food (this fish will eat virtually anything), it grows rapidly, and quickly adapts to low-light tank situations, when it will freely swim in search of food. It remains timid, however, and will quickly dart into one of its many lairs when it is startled.
I haven't been able to find any sites that list proper tank requirements though. But the tank should definately be at least 20 inches wide, preferable 24. They need adequate space to be able to turn around. This is of course provided that the fish grows to the 12 to 18 inch mark. Still, a large tank is needed because of the hyperness.