I use a 40 gal. breeder (sorry, panicked a while ago because I'm terribly OCD when it comes to my beardie), and it seems to work just fine. When I bought it, the man I got it from said it was 50 gal. If you're buying your tank second hand, or even right from the store, bring an accurate tape measure, just in case, and measure the dimensions. While I'm sure some people won't look upon this too kindly because it looks like you're questioning their judgement, it is better to be safe than sorry and waste all that money (and if you buy it second-hand, there usually are no returns). Sometimes the people selling them don't know what they're talking about. Again, this depends how big your beardie will get within the next few months. I think anything under 18" wide is just too cramped up.