Looks like I will be spending more time with you folks over on the parent forum soon. I ordered a 40" bearded dragon set earlier (no dragons yet, I can get them locally as my LFS actually breeds them so is able to offer really good prices on them). I know 40" is on the small side for adult dragons, but I can't fit a 48" one on the racking I have, and figured it's better than a 36". If they really get too big, I will just have to move them to a different room on a stand of their own.
Next on our species list, firebellied toads followed eventually by mantellas of some sort.
Depending on the other dimensions of the viv, a 40" could suffice. The 'recommended' viv size differs so much depending on who/where you get the info, but IMO a 48 inch is ample.
We are fully able to face the possibility of needing a bigger Viv one day though, I've made it very clear to my wife as a condition of having Beardies rather than leopard geckos. lol