Stay away from baby aro with yok sacs, but otherwise IMO no other size should be avoidable. As long as your willing to do the research, proper attention and care that baby arowana require, raising one is very do-able and rewarding. A baby aro should be raised with pretty much no tank mates. If you want a young one that is more stable, go with one in the 5-6" range, but this still limits tank mates. A 12"er would open up to a larger variety of exceptable tank mates.