I have the soft duffel bag (Adventure Tail Pack Bag A9510096) also and really like it. As framebinder stated, it is 50L.
My girlfriend lives 2 hours away, and I ride to visit her about every other weekend. This thing is perfect to hold clothes for an entire weekend and then a little more. It attaches/detaches very quickly with plastic buckles-so I can just carry the bag inside like my typical basketball duffel bag that I would take if I had my car. The other side of the plastic buckles loop onto the frame. You can see how they attach better in the manual here: http://store.proitalia.com/downloads...s/A9510096.pdf
I got mine off eBay for approximately $180, but list is $220. My only problem I have had is that I rode without the bag, but left the mounting straps on the bike, and the front right one (positioned above the exhausted) end up sitting on the exhaust and melting. I think I will replace it and use some velcro to mount it underneath so its out of the way because it is a pain to remove them each time, then risk misplacing it.