My goose down sleeping bag hardly takes up any space in my backpack when I have it snugly stored in this sleeping bag compression sack. Mine is the medium size, so 8 x 19 inches, but there are even smaller compression sacks if you have a higher fill down sleeping bag (the higher the fill, the more it will compress). There are also larger compression sacks if you have a synthetic sleeping bag.
This compression sack makes it really easy to get my sleeping bag down to the size of a football. There are top and bottom end caps that help squeeze out any extra volume. After my bag is safely inside, I put a trash compactor bag around the stuff sack to make sure that my down bag is protected from any water leaks or the elements.
You can also use compression sacks to store your down jacket as well. Remember to always unpack your sleeping bag and down jackets and let them air out when you get home – compression sacks are only intended to be used during a backpacking trip, not for permanent storage. Down needs to be aired out so it retains its loft, which is what keeps you warm!
Compression stuff sack for sleeping bag
These look very similar to the event compression drysacks being sold by sea to summit.
I replaced my alpkit compression sack with this one to gain water resistance and it paid dividends. On my last camp my water bladder burst and leaked throughout my pack. The bivi bag was fine, and the sleeping bag (down) was too, but it surely would have been ruined for the night were it not protected.