Remember that your down bag when packed will take up about half as much space as your Cat's Meow! You'll find that a partially compressed down bag will be quite a bit smaller than a tightly compressed synthetic bag. In other words, you'll find that your down bag will take up less space even if it isn't tightly compressed.
You can compress that down bag to less than the size of a soccer ball, or maybe even to the size of a softball. Whether you really want to do this is another question, because it will also have the rigidity of a soccer ball. IMHO, it's easier to pack when the stuffed sleeping bag has some "give" and can conform to the shape of your pack and whatever else is around it. Some folks don't even use a stuff sack or dry bag but use a waterproof pack liner and just stuff the sleeping bag into the bottom of the pack. They then put their other gear on top of the sleeping bag. I suspect this practice results in just as much compression of the bag as using a compression sack, but the bag conforms almost perfectly to its surroundings so probably takes up less room.
I personally use a 13L dry bag for my WM Ultralight (short). How much I compress it depends on how much I'm trying to cram into my backpack. When I arrive at my campsite, I set up my tent first thing and then immediately get out the sleeping bag so it has plenty of time to expand before bedtime.
The main thing is not to leave the bags compressed between trips and to try not to compress them tightly when damp. After each trip, fluff up the sleeping bag (I use the clothes dryer on low for about 3 minutes) and store it loose where it can breathe. Some use the large cotton storage bag provided–store on a shelf with nothing on top.. Some hang in the closet (just make sure the clothes each side aren't compressing it; a lot of us have pretty full clothes closets). Some store under the bed (advisable to cover with an old sheet to keep the dust bunnies off). Just be sure the space is dry and relatively clean.