Ditto what Bluefish said about the Atmos 50 on a weeklong trip. My weight for such a trip would be about 30 pounds; no more than 35. I also use a down sleeping bag, and carry down clothing for insulation. If your preferences run to synthetic bags and pile jackets, you may need that 70 liter pack.

Take your gear to the store (including the food for the length of trip, and a couple of quarts of water. (If you're going to have to carry extra, take that filled container, too - try the pack with and without it, since you won't be carrying it all day.) Do NOT let them fit the pack using sandbags - you need to know how YOUR gear fits the pack. (For example, will your water bottles fit the water bottle pockets? I've rejected packs when they won't.) Also, the weight distribution will be different between your gear and a sandbag, and you want to know how the pack will ride with your gear in it (unless you plan to backpack with sandbags instead of gear.)

You might call the store ahead of time, and ask them when they aren't as busy - that way, you can take your time, and you won't have a bunch of other customers tripping over you. The store will appreciate your thoughtfulness.

Also - have you looked at Deuter packs? The Osprey packs - especially the Atmos - don't fit everyone; Deuter is a top-notch pack with a great suspension that often works for people who can't use the Atmos. (I have the opposite problem: I can make a Deuter pack work, but it doesn't fit me as well as an Osprey.) Your pack, like your shoes, must fit YOU, not me.