Phil Maher wrote an answer about Outdoor Research UltraLight Compression Sack on February 5, 2014
You can pull off a 5L or go up to the 8L and have plenty of room in either. Your Summerlight long stuffs at 6x12" (WM specs are XXS in the stuff sack they provide), so a little shy with the 5L in the width, but more than enough in the length. If you really need to compress your bag, that's how it is, but over time, it's tough on the down and shortens the lifespan, not to mention that it requires extra time to get the bag lofted back up before bed. My WM bags are probably the one thing I never ever compress unless I absolutely have to, and 6x12" is already a very doable packed volume to begin with. Hope this helps.