It's size also depends on what gender you're boa is. But even then, if you got a female BCI (that's what I'm assuming it is) you're looking at around 7-8 feet, if that, and for males you're looking at even less. The 8+ ones aren't particularly common, and like yeroc1982 said, feeding schedules can also help maintain the size of your boa so they grow at a rate you and your girlfriend are comfortable with (without underfeeding!).

It would be a shame if you gave it up before you even knew if it WOULD get big. See if you can work out a compromise where you give it a shot and see how it goes.