With all due respect brother, you need to know this stuff before getting something like a Burm. This is elementary care. You should be reading non-stop with the animals you've acquired already and the questions you're asking. I think you're well intentioned but your eyes are bigger than your stomach. Learn THEN acquire, not vice versa.
Food item should be just slightly larger than his girth, and you want to get him on rats ideally if they're of the correct size. Burms can and will get hooked on mice if they stay on them long enough.