Hatchlings approximately 15" - 18"+/-. Females average 5.5', males average 4.5' adult size. Maximum size for this species is around 7 feet in length.
For adult Brazilians an enclosure measuring 48" x 18" x 16" will be sufficient. As rainbow boas are semi-arboreal, a vertically-oriented cage will allow your snake more room to climb & is something to consider if you have the space to accommodate this sort of setup. Remember that ALL enclosures must allow for a proper thermal gradient that the snake can utilize, with a hot spot on one end and a cooler spot on the other.