This varies a lot by the person. I now wear size 11.5 shoes just always, and before my first long trip my nominal shoe size was 10. It still is close to 10, mind you, I think my feet have permanently grown a *little* but I suspect it's more that I just can no longer tolerate really restrictive shoes.

But this is so individual. If you're buying boots rather than shoes (more expensive, last longer) then I'd suggest something like one full size up from normal, with socks to fill up the slop space AND only do this if you have the chance to do some decent hiking in those beforehand. Heck, you might even consider picking up some cheap shoes in that larger size ahead of time, shoes that feel about the same to you in size as the larger boots and first do some long walks in those --- to see if they'll work for you. Then buy the boots in that size if so.

Boots vs. shoes on the AT is a whole different issue. Most PCT thru-hikers wear shoes, but on the AT it's more of a mix with I think a tilt more towards boots than shoes. I used shoes on the AT, but some feel strongly that more ankle support is needed due to the overall poorer trail quality. YMMV a lot, I was happy in shoes but experienced people that I respect a lot feel that boots are the way to go on that trail. So go with what your gut tells you after you've done some training walks.