I bet you'd be very surprised if you rescued them and took real good care of them. For example, read the accounts Wednesday13 provides here:
http://www.monsterfishkeepers.com/forums/showthread.php?545336-Red-tail-cat-growth-rate/page2
It is often reported that even after severe stunting due to a small tank, catfish are still able to reach (if not full but not too far from it) potential given the right home and care. Surely it varies with species, specimen of the same species, and circumstances but these IDSs will reach 2' for sure. Their "full" potential is 3'-4' with wild records in the 5' range.