For my birthday I want to order an AdventureBilt fedora

As in the exact same one they used on the screen for Harrison. Size and everything.

Thing is, apparently Harrison's size for the original Raiders was 7 1/4 (58cm). I figure that it has to have expanded a LITTLE bit over 20+ years so maybe it's a 59.

Either way, when I measure myself with a measuring tape, loosely, I'm a 60. BUT if I pull it as tight as it can go I'm a little under a 58. So I think it could fit. Would it stretch eventually?

Sounds like a 59cm should fit you fine. Simply measure around your head where the sweatband would sit, and approximately how tight you want it to fit, and that's the size you would need. Don't get it too tight, because it won't stretch naturally. Besides, it would be uncomfortable. You can always put a hat stretcher in it to keep its size, but it is doubtful that the sweatband will shrink. Adventurebilt's are top quality, and made to last and keep their shape.