i stole mine from a friend who shouldn't of had him... he was underfed and in poor condition. He was very skinny and only about the size of my middle finger. In less than a year later He tripled in size!
All I know is I saw a few in Las Vegas a few weeks back that were easily 18" long and reall big around. It makes me question if they're actually appropriate for home aquaria.
If you have the right size tank they are appropriate.
My LFS has had them for long periods of time and everytime I go in they would be bigger. I even asked if it was the same fish and they said it was and my friend who works there comfirmed it.