The manufacturer's website will have the maximum recommended weight for each pack model (subtract 5 lbs. for comfort; usually the maximum is a bit overstated).
It's highly recommended that you acquire all or most of your other gear before buying your pack. The pack has to fit you, fit your gear and be comfortable for you hiking with your gear inside. Not everyone is comfortable in an Osprey pack (my hips and their hip belts are definitely incompatible). Pack fit is nearly as individual as shoe fit.
Please note W_D's comments on the bear canister, required in much of California (and in some places elsewhere). I have managed to get a bear canister (Bearikade Weekender) into my 40L pack, but it's a very tight fit, and the rest of my gear is really compact. That's the only time I've had to use the extension collar, so with the canister the pack capacity used was more like 50L. I also had to squeeze more items into the outside pockets than I wanted to.
Check out the articles on the home page of this site, left hand column
for more info on slecting a pack.