Biggest male Bci that I've had was several inches under 8 feet. Biggest female was several inches over 8 feet. Another male boa was under 6 feet as an adult, so they can vary tremendously. Parents' sizes are the best guide to a baby's potential size, as artgecko wrote.

I have fed a rabbit to a 7 foot boa constrictor once. Because I had a free rabbit and needed to get rid of it. Rats of appropriate size have been the staple diet.

There is much good information here:
Clay English, Ultimate Boa Constrictor Care Guide, https://www.madaboutboas.com

Good luck