What’s your budget? It is best to be able to try packs on and walk around in the store with weight in them, but a travel store in the mall may not let you do that and they may or may not know how to fit you correctly. If you have an REI or other outdoor products store they can fit you correctly. That said, you can do o.k. shopping online (and possibly save $$) if you’re of relatively average size and you read reviews of packs you’re considering. I’d look for something in the 50-65 liter size range (50 if you can make yourself pack light). Almost no one needs more than 65 liters. Some nice features include panel/side loading, a detachable daypack, and straps that can be hidden for when you check your bag on the plane. These features aren’t absolutely necessary, so if you find a top-loading pack with no daypack that fits well and is a good deal, it won’t kill you to use it for your trip.