@DTJC
I've a 13 litre Airlock dry bag and last weekend I got: An Alpkit Hunka XL; a PHD Minim Ultra sleeping bag; a Cumulus 150 quilt and an Exped Synmat winterlite sleeping mat (and inflator bag) into it and had room for a bit more, probably a thermal or two.
For information I've also an Alpkit tapered airlock for the seat post so 13 litre and a Wildcat Ocelot partial frame bag at 4.5 litres so that's just over 30 litres in total. More than enough for summer but I tend to need a backpack for the extra bulk of winter kit.
