The same LFS that told me aiptasia was a coral that I should spot feed is the one I went when I started my 10g and asked her about them. She said they can get the size of a basketball and a 55 would be fine. I'm sure that information is accurate.
Really though, they are cool looking fish, but I think you need 100+ gallons for em. =/
Washburn - Isn't there a puffer that also needs shells to grind down teeth? Figure 8 maybe? I can't remember :p