YingshellEither 7L or 10L should be fine. I tend to err on the side of using larger stuff sacks, knowing that they'll compress more once in my backpack (especially the quilt/sleeping bag, which I put at the bottom of my pack).
YingshellThat depends on your model, but I have a 7L Sea to Summit Compression Dry Sack and a 10L Sea to Summit Ultra Sil Compression Dry Sack, and both fit my 20º Long/Wide 800fp quilt well. If you have an 850fp new one, I would lean towards the 7L, but you can always cram other things into the 10L (pillow, hat, etc).
SkylerButlerGood point. Mine is wide. I think I might want to put my sleeping clothes and maybe a down pillow in there, too, some maybe 10L to be safe.