I took a backpack that will hold 3950 cubic inches. I think somewhere between 4000-5000 cubic inches is a good amount of space. Also I would recommend one with a large middle pocket and a smaller pocket on top and on the bottom. I found it really convenient to have the top pocket for maps, journal, sunscreen etc easy to get to. Then the bottom pocket was good for bathroom supplies so I didn't have to dig through my bag for that every night. Also reccomend bringing a small backpack as your carry on, it was handy all the time. I think ten outfits would be fine. We really did a lot of washing of one outfit in the sink and then hanging it up overnight. We also never had a problem with clothes being taken. We washed our clothes at one of the campgrounds, which are really little mobile homes, and no one touched it. Another time we did it at a laundry mat and the machines locked until the load was finished so you just had to make sure you were there to get it when it was done. I don't know if that happens at all of them though. Bring light weight clothes that will dry fast.