I measured it all out and drew it in Google Sketchup a few years back (I wanted to be sure I could fit my big telescope in the car with all my luggage!) - I worked it out as (approximately) 8.6 cubic feet with the seats up and 13.2 cubic feet (total, 4.6 cubic feet additional) with the seats down, keeping everything below the windscreen. Can't recall how precise I was, but the numbers sound about right (compared to my Beetle which is about 4.5 cubic feet for each compartment and carries about the same amount of luggage, without the telescope). I've attached a .png image file to this post with the dimensions.
Michael