Great question — answering as the UphillTech team:
You need two separate bags. Washing clothes requires something be mostly water tight. Storing them requires airflow. These two things are mutually exclusive. For washing, you need movement inside the bag. Figure out what size will just fit your clothes, then buy one size larger. For storage, I would buy one mesh bag that fits what you most often carry. The mesh will allow airflow and help prevent mildew and smells. Do not toss wet clothes in a bag. I recommend hanging them on your pack somehow. You look like a bum, but at least your clothes won't grow things....
Full answer by Russell Steen: https://outdoors.stackexchange.com/a/7780 (CC BY-SA 3.0).
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