Size

How big can a red tail boa get length and girth?

BCI or BCC? Either way, you'll find the answers in the sticky that details the differences between them.

I'm assuming you're thinking about a BCI, since BCC's get quite large..
On average, males get 6-8 feet, females 7-9 feet. As for girth, I couldn't tell you. I know they do take rabbits when they get large enough though.

It also depends on the locality... there are many varieties of dwarf bci's including but not limited to Nicarguan, Corn Island, Cay Caulker, Cay Crawl, Sonoran Desert, and Hogg Isle. Most of these stay below 6 feet, so would pretty much be about the same size as ball pythons, although not quite as heavy. If you're looking to get a BCI that doesn't get that big, you should definitely look into one of those dwarf localities.