I bought my Colombian female boa in September 2010 (she was quite small then. but I don't have the exact date of birth). She is currently about 5'6" and 6.2 lbs. And here I was wondering if she was too small for being almost three years old, compared to all the big female BCIs that I have seen.

As long as she doesn't look dangerously thin, she probably isn't malnourished. Boas grow at their own rates, and there is quite a bit of variability in size.