I would recommend feeding one appropriately sized rabbit (such as a 3-5 pounder) every 10-14 days (if this snake prefers rats, offer 3-4 jumbos over the same time period).....some snakes have higher metabolic rates and every snake will be different so you will have to learn what is the optimal diet for the animal while making sure you avoid the snake from getting too heavy.
Your burm will continue to grow. I have personally seen a 16' male but that is probably the exception. You can expect a male burm to reach 10-14' with half the girth of a similarly sized female.
As far as cages go, I would highly recommend that you keep this snake in a cage made specifically for snakes. I would recommend any of the following: Neodesha, Vision, Habitat Systems and Crescent Moon Creations. At 9' your burm should have, at the very least, a cage that measures 6' x 3'....that size cage will actually hold you over for the entire snake's life (if it ends up being an average sized adult male).
Hope this helps,
Rob Carmichael, Curator
The Wildlife Discovery Center at Elawa Farm
City of Lake Forest Parks & Recreation (IL)