Congrats, boas are awesome!

As was said, your average Columbian red tail female will get between 8 - 10 feet long, but they can get bigger. There have been some (and I mean like a number you can count on your hand lol) that reach more than 12 feet, but that's like seeing a 7 foot human. It isn't common, but it is certainly possible.

Also, most boas can still be satisfied with rats alone, you don't necessarily have to do rabbits, just feed multiple rats when she outgrows the largest size