I think you might be able to get that measurement by looking at the Euro Jazz and potentially other cars too. It seems like in Europe they list the cargo space behind the seats, up to the window level.
Why I believe this? Well, in the past few days I was looking at Euro sub-compact cars for my father and I'm trying to steer him toward Fit/Jazz. I can't believe I'm doing that, but the truth is it's a great little car if you can get past its looks, which I personally cannot, but he might be able to do that. Plus, over there it seems to be cheaper and better equipped compared to the competition. I guess different market with different strategy.
Anyway, looking at the Jazz’s specs, the cargo space with the rear seats up is listed at 353L (12.5 cu.ft.) and with the seats down is listed at 845L (29.8 cu.ft.).