Geoffrey Buck wrote an answer about Sea To Summit Compression Sack on April 23, 2010
Depends on the bag. Bags meant for colder conditions have more insulation and are therefore bigger and bulkier. Looks like EMS lists the... "stuff size" on their website (under technical specs) if you can find your bag there. I'm not sure if that corresponds to the dimensions of stuff sacks listed here (I'm used to manufacturers listing the volume), but it's a good bet that it will give you an indication of what size to get.