I would say that they can get up to 12" or so, with females being a little smaller, about an inch or two. It depends on how long you intend to keep them. Average for most would be 5-8" male or female, but they easily grow past this size, that is if you pamper them and keep them very healthy. I've seen some very large adults that get traded in to a LFS here. I have also seen oscars pushing 15", but they are rare.