For an adult boa, it's going to be a minimum of 5 feet long, 2 feet wide, and 2 feet high. You should get a tub or a custom cage, that is plastic all over but has glass doors at the front. Don't plan on keeping more than one together.
If you get a small subspecies, like hog island for example, you will be fine with feeding it large rats. However, the common boas, especially the females, are going to need something as large as rabbits eventually.
I know boas are fun, but do not get one if you aren't 100% sure you will be able to care for it at adult size. Their food is going to get expensive, as is their housing.