I'm partial to the smaller boas.
I like the CA Motleys, but that's sort of expensive for a first boa.
I also like the Hog Island boas. I have seen some that were "HUGE". These were well over 7 feet long. I've also seen some 10 and 12 year old Hogs that were under 5 feet. Both males and females are capable of going to one extreme or the other.
A male colombian is a good choice, but they are also capable of getting "HUGE" if fed large amounts of food. I've seen Colombian males as large as 8 feet, but I've "heard" of larger ones. I've never seen one over 8 feet. Not even in pictures, so I don't know how true it is of 9 foot or larger ones.
If you want a boa, but want one that stays small. I'd go with a Tarahumara Mexico Boa. Those are suppose to be "true dwarfs". At least, that's what I've heard. They're moderately priced and stay small.
I think Gus at Rio Bravo Reptiles has some of the nicest ones, but other people have them as well.
Just shop around and learn more about the different types of boas before you decide on the one you want.