The largest black I've read about and seen was a 17" one, but people claim to have seen 30" ones, now if this true they get slightly bigger then the common, which in the wild get to 2' and there are documentaries on you tube with footage, the biggest common I've seen in aquaria was Liams 20" one. If you are looking for a smaller wolf, look into erythrinus species, if you can't keep a common or black, I wouldn't recommend a gold which though gets smaller staying more like 16" is much more active which means it requires the same size tank imo. What it lacks in size it makes up for in activity. There is erythrinus erythrinus, erythrinus sp peru (these are the two common ones), erythrinus sp kessleri, and ery. sp tahuayo that stay small and fairly active compared to commons, and blacks ime.