15 is a good size. I actually use one of the small exo-terra vertical terrariums, but those are orver rated (and way over priced). You'd be better off buying one of the same shape that is sold as just the terrarium (not in a kit like the Exo-Terra). I believe Petco in the US sells the kind I'm talking about that is not sold in a kit. The brand is Zoo Med I think? I believe I link it in the Gray Tree Frog article.