How big will he get?

At 5 months old, Midnight was 5.7 lbs he's a little bigger now. At 6months he's 15inches from shoulder to hip and stands 10inches tall at the shoulder..

given that he was born in a woodpile, the best knowledge I have of his parentage is a guess from a distance that 'mother' looks to be between 9-12 lbs, tall long and sleek-a tortie beauty who won't let humans withing 40feet.

I'm curious to think about how big my little guy might get when he grows up?

Cats grow at different rates so it's nor really possible to say.  However, taking a stab, he's a similar weight as my female Oriental kitten so I suspect he will be average size, not large.

Yes, it really depends on the cat. Once he's around a year he will likely be about done growing though, so you'll have a very good idea in just a few more months