what size would be right for storing a 30...
what size would be right for storing a 30 degree down sleeping bag?
I believe when storing your sleeping bag, the bigger the better. This allows the down to be as least compressed as possible.
You could get an XL here but thats still a bit small (20l). I'd get an XXL (about 30L) which isn't available at backcountry but they appear to have one at summithut.com.
Or just grab on of the huge blue bags at IKEA, they're plenty big.