I was originally planning to get a 67 gallon and now it seems I may not be able to. I wanted to know if its really bad to consider a 40 gallon? If so, what's more important for beardies: the height or width of a tank?
I believe so. I've kept adults in 40g breeders for years and years and never had issue with my dragons being happy.
It's only been recently where I've gotten more space for my dragons that I started building some of them 4x2x2s.
Many of us start out with 40BRs... it's great to be able to upgrade and give your dragon more space, but yes, a dragon can live a happy life in a 40BR, especially if you're good about giving him or her daily walkabout time for a good stretch. As ClaudiusX said, for years, 40 was the standard size. Going bigger is really one of those "it would be great if you COULD" things, but I do believe you can provide enrichment and quality care in with a 40 as well.