Do you have the rest of your gear?
The "norm" is generally between 3,000-4,500 cubic inches (depending on how long you go between resupply and what your personal weight threshold is). What you'll want to decide first is what all you need, at a minimum, for a safe and enjoyable excursion. Make a list of "Necessary" and a list of "Like to Have". It's up to you, but generally the "Like to Have" list will be left behind.
Peruse the websites for Osprey, Granite Gear, Gregory, ULA, Deuter, GoLite, Arc'teryx . . . . . I know I'm forgetting a few, but anyways check to see where they are sold, and then go visit said outfitter for a proper fit. They'll have weights for the bags and usually an incline, so you can get a feel for a full pack and your center of balance. I personally bought (sight unseen) an Osprey that just didn't fit me right - finally drove out to Chicago, tried on everything in the stores, and finally settled on a Gregory. A pack is something you should ALWAYS thoroughly try before you buy.