There are two different Blacks going round at the moment. The Xingu which appears to wider spread than just the Rio Xingu. These guys will probably max at about 12" - 13" in captivity and a little larger in the wild, although not confirmed upto 18"
The second is a wolf sold as the Venezualan black or Hoplias Macrotphalmus in Europe. This is a much larger fish reaching mid 20" in the wild. The biggest captive one I have seen is 14".
There are bigger Wolves. Both lacerdae and aimara can reach over 30"
we really need some experts on wolffish, i have two mala's and one lacerdae soon to have a xingu black from brazil, jelly g from shark aquarium said that the xingu black from brazil will get aimara size