9L for shelters and sleeping pads
9L one fits well in my ultralight backpacks, and into the external mesh pockets.
I put my shelter - DCF tent or tarp, regardless of the size - into 9L sack and put it into mesh pocket. It is much easier than trying to roll it down tight and put into sack that came with the shelter which is usually very tight.
I also fold (not roll) my sleeping pad (and often bivy too) and put them in the sack, which I slide into the backpack, closest to my back, which prevents hard objects (like pot) from digging into my back.