Recommendations are just guesses. The only way to know if a given size is right is to go to the store with all the gear you need for a given trip, plus the food you'll be carrying (or an approximation), plus the largest amount of water you'll carry. Load it into the pack or packs you like, and walk around the store a while. (Call ahead and see when they aren't overly busy; you don't want to be spreading stuff all over the floor when they're crowded.)

As you look at packs, pay as much attention to the suspension as you do to the volume. Get too skimpy a suspension and you'll suffer for it every day, all day. (Another argument for loading it up and walking around for a while.)

I know folks who could travel for a week with a 50L pack; I can make a 60L pack work for that. Nowadays, there are just too many variables to deal with to make a blanket recommendation that's accurate.