It looked like maybe 6" or 8".
Sadly, if this did reach mass production in anything close to its current form, I doubt it would find many buyers even at an economy-car price. It deviates too much from what people expect a "car" to be. This has Cd 0.189; Hucho mentioned he expects production cars to reach 0.20 "and that's it," and I'm inclined to agree with him.
That said, I would give my eye teeth to have one of these.