So I'm looking to get my dragon a bigger, better enclosure, but I'm not sure if I should go for tall or wide. There are lots of different options here:
The absolute minimum for an adult bearded dragon is 36x18x18 I know several people who have housed their beardies in these and they are perfectly happy and healthy. I think the exoterra's is 36x18x16 which is ok because you don't need too much height with a beardie but you will have to adjust temps for the shorter height. Bigger is good but a 40 gallon breeder (36x18x18) is just fine.
A 40GB is too small for an adult. The length is ok but an adult beardie will be longer than the width of the tank... it's too small. 4X2 should be bare minimum.
There are tons of places to order custom tanks from. Here are some of my favorites (in no particular order),