If you are going solo or with someone is another consideration. If you are solo and not using a tent, a smaller pack might be feasable, but if you are packing a tent, stove, food, water pump, clothes, and the various other little items, a 50-60 L pack will serve you well for 1-X nights and give you the most versatility. The most important thing I have experienced is having the pack fitted at your local REI, a well fitted pack makes everything better.
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