Well, Chris, first thing I'd do is double check to be sure the guides really carry the tent and food! That would have a significant impact on what you need, pack-wise.
That said, I would think that a pack with a capacity around 3,000 cubic inches would give you enough room for your gear—clothing, camera, and the like—as well as a little extra space in a case you need to pitch in a help with the party's load.
Your shop may have been referring to the Osprey Talon 44 ($159), a light backpacking pack that's suitable for trips where you're not carrying all of your own gear. It's a lightweight (2 pounds 10 ounces), comfortable pack that has 2,700 cubic inches of space inside. So, not a real big pack. But then, nature abhors a vacuum, and will find a way to fill whatever space you have.
Gregory's Z45 ($189) is a similarly sized pack that's a bit heavier-duty than the superlight Talon. The Gregory weighs a pound more, which won't really be felt on your back but shows itself in sturdier materials and a little more load-friendly suspension.
If you think you might need more capacity, 4,000 cubic inches would be the next step up and would give you more room without creating a huge pack. I really like REI's Flash 65 ($169), a well-designed light backpacking pack that is right at the size I just mentioned. It carries well, has lots of packing options, yet still weighs 3 pounds 2 ounces, so won't be a big burden to stuff into your luggage.
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