There's various subspecies, which all vary in adult size. It also varies with gender, as females generally get larger.
IMO, the average, plain-jain Columbian boa (often called a columbian RT) usually averages around 8-10 ft in females, and males around 6-7. The "true' RTBs (like Surinames, Guyanas can routinely get closer to 13-14 ft, like what you read. And the Hog Island boas are often popular because of their much smaller sizes, probably closer to 7 ft for an adult female.
I'm sure the RTB experts on here could be more helpful.
Hows your boa doing so far? (You did say you have one, didn't you? Hard to keep track sometimes...)