The size of the backpack very much depends on the volume and weight of the gear and food that go inside. That's why it's a good idea to buy your backpack last, after all your other gear! If the pack is too big, it won't balance right and you'll be tempted to fill it up with stuff you don't need. If it's too small, you'll have to tie stuff on the outside, also affecting the pack's balance and making it easy to lose the tied-on items.

Check out the articles on the home page of this site, left hand column. They will give you an idea of what to look for when gear shopping. The most important measurement for packs is your torso length, which has little to do with your height and weight. The articles will tell you how to have someone measure it.

The most important thing about packs is fit, fit, fit! The pack must fit you, it must fit your gear and it must be comfortable for you with all the gear (including food and water) inside. Pack fit is almost as individual as shoe fit, which is why you should take recommendations with a grain of salt.

Osprey makes nice packs, but not everyone is comfortable in them. The ones you mention may be heavier than you need--it all depends on what you're putting inside!