The most important part of a backpack is fit, fit and fit. It has to fit your body, it has to fit your gear and it has to be comfortable for you while carrying the gear inside (totally different feel than with sandbags or other weights). Backpack fit is almost as individual as shoe fit.

That's why it's strongly recommended to get all your other gear before shopping for a backpack. Add the equivalent in weight and bulk of a week's food and a day's water, box everything up and take it to the store (or have it ready to try on when you receive your mail-order pack, and hike around the house for a couple of hours, while you can still return the pack).

The articles on the home page of this site, left hand column will give you lots of tips on gear selection, including your pack.

I could use a 35 liter backpack, too, if I didn't take any food. For a week out, I need 40 liters. Your (and everyone else's) Mileage May, of course, Vary!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey