The bigger the better for beardies. My father and I custom build all the enclosures for my reptiles so we have come up with a few nice ones! For a full grown adult male, which can be rather huge, I would suggest 3-4 ft long by 2x2, that's what I kept my large male and he loved it. Beardies also can have a pretty simple cage. Really the bigger the better, with more floor space the dragon has more room to hunt and sprint towards their prey which is awesome to see! You don't want a 22" dragon knocking his face in the glass attacking the crickets. I think for a juvenile a 40-55 gallon is fine, maybe even for an adult in some cases. I however prefer to spoil my animals with room. When I was younger and had several beardies I often fantasizes about converting a whole room in my house to a dragon sanctuary! I've seen it done, ah well, pipe dreams!