Find something in the 30-40L range. You will be surprised how much gear you end up carrying as you start filling out the kits for your pack.
I am using the ortovox haute route 32 currently and it's my favorite I have used to date (mammut 22L airbag, BCA airbag, dakine mission, and Patagonia ascensionist). I have a buddy using the ortovox peak 45 as his guide pack and he is in agreeance that there's no complaints with their fit or spatial management. Ortovox in my opinion is doing it right and I think you can find some stuff on deals right now with the end of the season.