Cage size is fine, gives her more room to move around, just put a water bowl and maybe a hide in it. As for the diet, I also have a slighty overweight
8 1/2' female boa, and I tried putting her on a little diet. Well she wasn't too happy about it and I got tagged pretty good on my left hand one day when i was going to take her out.

P.S. Lets see some pics!!!