For compression sacks, you can always try to get the smallest one possible to still fit your gear. However, I always find it's just easier to have a larger one that you don't have to fight. Just through your stuff in and let the compression sack do it's job.
I think I have the 20 L stuff sack (with the Event on just one end for air to escape) for my 20deg extra wide down quilt and my full length thermarest Xtherm, and there is plenty of room. I am lazy (aka efficient for some), so I just roll the quilt and air mattress both up together and place in the stuff sack. But as said above, it is much nicer having the bigger stuff sack to start off with to get everything in, and then roll the top of the sack down slowly compressing everything into about 1/3 of the original starting volume.
X2. I have the xl event. Plenty big to easily stuff a 0' quilt, thermarest xthem, borah bivy, and inflatable pillow. Stuffs into the bottom of the pack easy and if I'm loading stuff it compresses more. Super easy. I would recommend the light weight material.
Also I cut the compression straps off as they were unnecessary and compressed the wrong direction if you're putting it as the bottom of your pack.
I'll add one more point that Ruskin hit on. I use an XL eVent most of the time. I think that's the size at least off hand. Anyway, I actually took the compression lid and straps off of it. I just stuff it full as others have mentioned and then roll it down a bit but not to the point that it's super compact. That way, when it's in my pack, it can compress and fit into all the random nooks around other stuff. The eVent will allow it to "breathe" and compress as necessary versus a regular dry sack that will trap air like a balloon. I also don't like compressing things down in smaller sacks because they become hard little footballs that are awkwardly shaped when trying to fill a pack and you always end up wasting space.