Getting this guy in trade, what do you think?

[Evenstar]

If it's a normal BCI male (which he looks like he is to me) then you're looking at him being around 5-6ft and around 20lbs as an adult. There ARE cases of BCI's growing larger than the average, but they're definitely not the norm. An appropriate-sized rat every 2 weeks will be just fine for him. He'll end up however big he'll be no matter what, it's just how long he takes to get there is what the feeding schedule/diet does.

This is absolutely correct - all of it.

Your boa is a normal BCI (not a "true red-tail" which is a BCC). Male BCIs will generally not get more than 7ft and 6 is the norm. Feed him every 2 weeks. You will love having a boa - they are so much fun!