Mine are 36 x 18 x 18 Vivs and I currently house all my corns seperatley my biggest 5ft male to my yearlings all have same size vivs, all my hatchlings are in faunariums. Eventually I will have some same sex living in pairs thats why I went for the size of viv build. I would probably have something a little wider personally but thats me. I think it would be a good size and you shouldnt have to upgrade.
Looks fine for a yearling but may need something bigger eventually - depends how big it grows.
For this viv you could get a piece of melamine - say 18"x10" - and put a shelf in there. it'd increase the floor space by about 40% and encourage him to climb - probably giving him more excercise than he'd get in a slightly larger viv anyway.
You could get away with that heat mat - but 11x11 mat would be better - you'll definitely need a stat though if you don't already have one.
Don't forget to cover the mat with something (eg. a large tile) to lessen the possibility of burns. Also a good idea so seal up the terminal block with low-tack insulating tape or silicone sealant so that moisture can't get to the electrics.
I used to house all my adults in 36x15x15 without any problems.The only reason i changed it was i had so many corns that to keep them in vivs would have taken up the whole house let alone the one room i have.
That size viv would be ample room for a 6 foot corn and you could also put in an opposite sex for short periods of time for breeding perposes