For a 10 day trip, my 40 liter pack was ample. I didn't use the expansion area at all.

Have you looked at the gear lists and other articles on the home page of this site, left hand column? It sounds as though

In addition to your gear, you need to include the equivalent in weight and volume of the food and fuel for the amount of time you'll be out. All that's going into the pack. Also your water (which will vary according to the frequency of water sources).

Two weeks without resupply is more than most backpackers want to do because the first few days will be really heavy. If there's no way to resupply mid-trip, then plan short days for the first few because your load will be considerable!

And please do follow the advice already given and get all your other gear before shopping for a pack. The most important things about a pack are fit, fit, and fit. The pack needs to fit you, fit your gear and fit you comfortably with your gear inside.