Danyal-
The tadpole madtom (Noturus gyrinus), which is the most commonly available, gets to about 3-4" in length; even so, they are seldom bothered by larger sunfish due to their nocturnal habits and sharp, mildly venomous pectoral spines. You have to be careful handling any madtoms (or North American cats in general). The largest madtom is the stonecat (N. flavus), which reaches 10-12" in length; they are also sometimes available. Several other species show up now and again, most of them in the 3-6" range. The pygmy madtom is less than 2" in length, but it is threatened in the wild and unavailable in the pet trade.
WWG-
The madtoms are actually the largest genus in the Ictaluridae (North American Catfish family).