Will is right about the 40L pack being way to small. I carry a 75L pack that expands to a 90L pack. It is great to have extra room for the all the stuff you may buy to bring back. It is personal opinion though on the pack you buy. My new pack is a North Face Pivotal 75 and I love it. When you go to get your pack look at all these things to choose the size:
* How much money you have to spend
* How long you are going to be gone
* How much walking are you going to do
* How much stuff are you going to bring with you
If you want any more help email me and I will send you some more stuff on buying a pack I got from North Face when I bough my new pack.