It looks quite small - how much protection does it give?
A surprising amount. That said, it's not a full touring screen, so please don't expect it to perform like a Honda Pan European! When you're riding a naked bike, your body provides a big target for wind blast. Flyscreens work by taking most of the wind from this area, giving your body an easier time. You still get wind around the helmet area (but that's why you bought the bike, yes?), but you don't get the fatigue in the same way.