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
"Parents' sizes are the best guide to a baby's potential size", i disagree with this statement. The father of my 17 months old BCI was 5,30 feet and the mother was 5,60 feet and my boy is 4,80 feet at 17 months old.