I agree with Warren, no need really.
Rabbits are extremely lean, low in fat... this means two things, they won't make your boa gain weight faster, if that's what you're wanting, and they would essentially take longer to digest, because of the extremely high amount of protein, so feeding as often as you would rats wouldn't be the best thing.
To answer your question, like Randy said, I would start with either two smalls or a medium, if you really wanted to feed rabbits.
I would say a small would be anywhere between 2-4 pounds, and a medium more like 5-7 pounds.
I remember when I lived in Miami 12 years ago I was getting 12-15 pound rabbits for my 15-16' burms.. lol..
I would find them at farms in homestead..
I always bought bunnies for my boas at the local livestock dealer, where one would get fresh fowl (chicken, goose, quail, etc.).
All the best, Joe
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Thanks and take care - Joe Rollo
'Tis not the stongest of the species that will eventually survive, nor the most intelligent, but the one most responsive to change' Charles Darwin