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Go to any REI. The people who work in their backpack department are excellent in helping you find a pack that fits. Plus, you can try on a variety of bags. If you're not in U.S. , try any large outdoor store.
Gallon size freezer bags work as well, are more flexible to pack, and can be compressed before sealing. Not to mention being an order or magnitude (or is it two orders of magnitude?) cheaper.
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