When I began lightweight hiking, I was able to pack 6 days worth of gear in a 3,100 ci pack. I had to lash the sleeping pad outside the pack and squish clothes & my sleeping bag into a compression sack, but it fit.

For people who are relatively new to lightweight backpacking, I'd recommend either ULA's or Six Moon Designs' packs; their larger models have frames, a good feature for people still cutting back weight. Gossamer Gear has some packs worth looking at too.

If you want to try the packs on first, your best bet might be REI, Granite Gear, or Golite brand packs. They're easier to find.