Not sure if your looking for a day pack or a something for multiple overnight use. We have been flying off to the NP's for years now and have had a few 40 liter sized packs. They fit well inside the overhead storage bins. We have Deuter act lite 40+10's that will work if not using the expansion sleeve.
The size listed by the airlines is very accurate for carry on luggage. We will day hike with the packs as well as use them for carry on's because we don't want to bother with another pack. We usually take a few more items on a hike in territory like Yellowstone than for local area hiking. We were hammered by a thunderstorm passing directly overhead last time there and were glad to have full set of rain gear. That was one I will never forget. Make sure you buy your Bear spray and have a wonderful time. Yellowstone is our favorite, you will love it.

jimmyb