If you are set on a columbian BCI, I would recommend a male. They don't get as large as the females and tend to stay 5-7 ft.

Otherwise, a hog island bci or a rainbow boa are excellent choices. So are dumeril boas - they make wonderful pets!

If you really want a giant boa, you will have to be responsible and plan for the future. There are options other than an overburdened rescue. Perhaps you have a child or nephew/ niece that you can "train" to one day take your snake once you reach an age that keeping larger snakes is not practical. Just a thought...