I think that one's about 20/25litre. It's just the right size to take my tipi, or before, the Vango Tempest. I've not tried it yet with the high front guard, may have to fit the low one back on, or make a rack.
Maxwell, there's clean clothes in there as well. Nothing nicer than pulling fresh socks on in the morning. Well there is, but socks are pretty nice.