Dan, although that care sheet is great, I'll add my $.02 as well.
It really depends on a few things
Sub Species- For example, there are some localities of B.c.i. that get no more than 4-5' while some other locals if fed a lot, will reach 8-9'+ and 40+ lbs. Some B.c.c. have been said to reach 12'.
Locality- In general, isolated populations like island locals are USUALLY smaller which may expain the 5'-6 1/2' Adult size for the Hogg island boas or the 4-5' max for a few other island locals
Food- Basically its common sense, the more the snake is fed, the bigger it will get. The bigger it gets, the more prone it is to problems associated with obesity. Every 2 weeks is fine for a boa over about 1-2 years. A 5-7' Boa of most Sub Species is healthy, Period. Any boa that gets much over that 7-8' mark is or has been over fed at some point.
These are the main points I can think of, If you have any questions then please do PM me
