Help with Feeding for New Boa

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In about a month - 6 weeks I am aquiring a female salmon boa (I heard wrong, for those who read my post on the enclosure forum).

She is 5 1/2 feet long, 2 years old, and very tame with an excellent feeding response. I am not sure of her girth (haven't measured her). Right now the people are feeding her about 2 large rats per week. They have said that they know she should be on rabbits, so I am buying a few from a local breeder. My question is how big should the rabbits be, and how often should she be fed these? I was thinking they should be about 3-5 lbs, but just want to be sure. I am wondering if she should be allowed to eat a rabbit whenever she acts hungry, or if a stricter schedule is warrented.

I appreciate anyone's input I haven't had a larger snake before (though I did work with a burm previously), and just want to have everything ready before she comes home (hence why we are waiting, they have everything for her, but we are setting up a new enclosure at our new place). I greatly appreciate it!

Thanks in advance!

TK