I don't have a local store I can easily take these to, and I was wondering what size Sea-to-Summit dry sack I should buy if I want to be able to store my big agnes crystal sleeping bag (long) without taking out the insulated aircore pad out. I'd like to go with one of these:

http://www.rei.com/product/730877?ext_cat=undefined

but I don't know if I should buy the medium or large...