I did some research on this when I was getting my custom cage made for my baby dragon. I believe 48" x 24" x 24" is the average "large" sized enclosure people reccomend. The one I had built is 72" x 24" x 24", and it looks huge. My baby dragon, Rygel, was far too small for it when it arrived, so he's in his "baby" cage at the moment, but we'll be moving him into his big home sometime in December.
The most important things to consider are width (needs to be wide enough to accommodate the whole length of your beardie, and allow them to turn around without bashing their tail). I believe ideal length is anything from 36" and up, and height is probably best if it's not too high as it could be harder to get temps and UV height right. I believe 18" - 24" is about the average for height.