You're dead to me!! A burm over a retic??? Dead to me...
With that said, I would personally plan for a 10x3 cage. You're looking at probably a 13' snake. So honestly a 10x3 would be a minimum. As for height, burms are a heavy bodied snake so whatever you use for enrichment be in branches or shelves, they will need to support probably 100 lbs. I swore i heard of large females getting to 200 lbs.
I would probably do 10x3x2 if it was me and possibly plan for larger later on. 2' is easily heated while still providing decent height assuming you keep the room fairly warm.
And with all that said, if you are set on a burm, at least get a caramel or albino labyrinth since those are the 2 i want and i can then hate you less by living vicariously through you with the snakes![]()