As you thought ditch the heatmat. Some use them but it would appear you don't need to plus if used incorrectly then they can be detrimental you a BD's health. Regarding size you're right bigger is better but be sure to choose a size suitable to your bd. does he climb high a lot? Does he run back and forth a lot? It doesn't have to be long and shallow as long as you have the minimum floor space.
I have a 6x2x2, its brilliant, but with hindsight I'd have a different size. Obviously the 6ft length is great and if you're thinking of incorporating a fake rock build then you want the extra foot as your walls will obviously take some space. If I could I'd increase the height and depth by .5-1ft. Once you have a floor/substrate in and your bulb you loose a good .5 of a foot from the height, if you go for 2.5-3ft in height then you can have plenty of rocky ledges and deep substrate areas for burrowing. In the wild they often burrow to regulate their temps and humidity. If you can, allow them to do the same, may need a deeper bottom plinth. Depth is definitely hindered by a fake rock background if only 2ft. I haven't measured it but I estimate my depth is now about 18-20" in parts due to the fake rock wall. Don't get me wrong this is enough, he has no issue tearing around but I can't help but think we would both appreciate an extra .5-1ft and it would just look better.
Good on you for going for something bigger than the usual 4x2x2, in my eyes this is to small. Remember you don't not have to conform to the norm, aslong as you have the useable floor space and the correct temp gradient and uv exposure it doesn't matter on the dimensions.
*FOR SALE* 3 female Rankin Dragons and 6x2x2 fully custom stunning vivarium. Thread available here