Well there are some small boas like brazillian rainbow boas, sand boas, and rosy boas. These don't look like the big boas at all though.
All of these stay very small... Smaller than ball pythons in most cases.
I'm not sure I'd call a BRB "very small." Adults are typically 5-6 feet long, with some reaching 7 feet. Granted, they're slender, but IMO they're in a different size class than sand boas and rosy boas.
Mountain bikes are for slow people, and reptiles are far better pets than cats & dogs!