Ups, maybe I should have kept my mouth shut...

I don't really know about the size I just found a link on another thread on MCM and they seemed a good choice so I ordered without being asked by the vendor about any specific size choices. I think, if extended, the combined capacity is 57 l which is a lot. We were riding for 4 days and did not even extend the bags. I think they are not 100% water proof but come with rain protection that can be put over them.

They have some flexible plastic sheets that come with it and are fixed to the bottom interior which keeps them in really good shape.