Backpack capacity will be an important consideration when choosing a pack.  If you will be taking only short weekend trips, a smaller capacity pack will work for your needs.  If you plan to travel for five days or more, or plan to be in areas where you will need a bear canister, you will want a larger capacity pack.  A general guide is:

• 1-2 nights (weekends):  20-50 L
• 2-3 nights: 50-60 L
• 2-5 nights (weeklong): 60-80L
• Extended: 80+

If you plan to do a variety of trip lengths, choose a pack appropriate for the longest trip you’ll probably take.  My 65L REI Ridgeline pack works fine for 4-5 day trips when splitting up items like tents, food and other shared gear with my boyfriend.  It also works for weekend solo trips.  A good rule of thumb is that a pack can always be compressed but you can only stuff it so full.