Obviously the quick answer to the question would be, "Large enough to carry all your gear." But, I'm stumped at the moment because I don't have all the gear I plan on taking (don't have the proper sleeping bag, don't have the proper stove, etc). I have a Kelty Redwing 50, which I think may be large enough to hold everything, but if that's the case, I think it will be a tight fit.
I also have a Kelty Coyote 80. This pack would certainly be large enough, but it's two pounds heavier than the other pack. The big advantage to this pack is the external frame…it fits me so much better, which can certainly make the extra two pounds worth the weight. Yet, I'm wondering if I should this pack is so large that I'll just be humping a lot of empty air in the pack.
For those of you who have section hiked or thru hiked the AT, what pack did you use (make/model/size and internal or external frame)?