Depending if your bag is synthetic or down, synthetic is more bulky and doesn't compress as well as down bags.  


measure your bag in the stuff sack it came in and you will be able to get a good idea for what size compression sack will work according to the specification list. 


A simple calculation you can make is to take the packed dimensions from the product page, multiply them so you have the volume in cubic inches then convert that to liters (as easy as plugging in cubic inches to liters in google!). This will give you an idea of what size the bag is slightly compressed.