The rule of thumb that I have read, is that their enclosure should be wider than they are long from nose to the tip of their tail. I know the water dragons have long tails, and to continually swing around without hiting their tails on the walls of their enclosures could make them happy, comfortable lizzies. Just my opinion though, even though they LOVE their water, for me, I'd prefer their tank to have more land area than that. So I would prefer a water bowl taking up no more than one-third of their enclosure. But that's just me, and I only kept young EWD's so the room didn't look so cramped compared to if they were larger.