Dragons are considered semi-arboreal; they do climb and can end up pretty high in trees. In captivity to much height is probably risky (for falls) but 24" is a pretty good height IMO. This gives room for climbing furniture and branches and such - for a bigger dragon the branches/rocks need to be bigger too and a good rock or two can take up a surprising amount of space. You could also use the height to build levels and add more interest/enrichment structurally. Just a thought though.
48x24 foot print is a good one, being closer to standard lumber you shouldn't have to make quite as many cuts, but that's up to you and how much space you have for the final enclosure. If you choose to go with the 15" height I would suggest cutting a hole for the basking light so it can be mounted in a dome outside the cage. My biggest complaint with my 4x2x2 was the basking bulb eats up space quickly and made it hard to get close enough to the UVB.
Front open doors are awesome - I couldn't go back, that's for sure.