All subspecies are referred to as "boa constrictors", while the nominate subspecies, B. c. constrictor, is often referred to specifically as the "red-tailed boa". Within the exotic pet trade, it is also known as a "BCC", an abbreviation of its scientific name, to distinguish it from other boa constrictor subspecies such as B. c. imperator, which is also regularly, and erroneously, referred to as a "red-tailed boa" or "common boa".
The boa constrictor is a large snake, although it is only modestly sized in comparison to other large snakes, such as the reticulated python and Burmese python, and can reach lengths from 3–13 ft (0.91–3.96 m) depending on the locality and the availability of suitable prey. Clear sexual dimorphism is seen in the species, with females generally being larger in both length and girth than males. The usual size of mature female boas is between 7 and 10 ft (2.1 and 3.0 m) whereas males are 6 and 8 ft (1.8 and 2.4 m). Females commonly exceed 10 ft (3.0 m), particularly in captivity, where lengths up to 12 ft (3.7 m) or even 14 ft (4.3 m) can be seen. The largest documented non-stretched dry skin is deposited at Zoologische Staatssammlung München (ZSM 4961/2012) and measures 14.6 ft (4.45 m) without head. A report of a boa constrictor growing up to 18.5 ft (5.6 m) was later found to be a misidentified green anaconda.
The boa constrictor is a heavy-bodied snake, and large specimens can weigh up to 27 kg (60 lb). Females, the larger sex, more commonly weigh 10 to 15 kg (22 to 33 lb). Some specimens of this species can reach or possibly exceed 45 kg (100 lb), although this is not usual.