When my female albino was about 4 feet or so I started offering her 1 to 1.5 pound rabbits. A jumbo rat is just shy of a pound anyway so you can start a 4 foot or so boa that's already eating one or two jumbo rats a 1 pound rabbit. Another female that was about 5 feet around August I started giving 2 pound rabbits. It wasn't long before both boas were on 3 pound rabbits! Now they're both fasting and breeding.
My big 8 foot female I give 4 to 6 pound rabbits to once a month. My 6.5 foot female I give 4 pound rabbits to once a month. When the two girls I mentioned above give me litters they'll go to 3-4 pound rabbits.
I avoid any rabbits over 6 pounds because after that all the extra weight is just all fat! A big breeder friend actually skinned a 8 pound rabbit once to see what it looked like underneath the skin. The entire body was coated in yellow fat you could hardly see the muscle!
PS: Danktat - How many times do you get: "Hey do you know you look like that guy from Scrubs?"