A subadult BCI (Boa constrictor imperator)
This individual is around 5 to 6 feet, but may reach eventually reach 13 ft (depending on the individual and the husbandry)! There is clear sexual dimorphism seen in this species, with females generally being larger in both length and girth than males. The average size of a mature female boa is between 7–10 ft, whilst it is 6–8 ft for the males. It is common for female individuals to exceed 10 ft, particularly in captivity, where lengths of up to 12 ft or even 14 ft can be seen.
(Photo taken from here)