Well, I tested out my new Cal Sci medium +2.5 today. I liked it quite a bit so far. I am 5'10'' or maybe slightly taller. I rode without the helmet to get a real feel for the wind difference over stock. I can say with the Cal Sci shield fully down there is less wind than the stock. Fully raised, but with the seat in the low position, i look through the shield if slightly slouched or if sitting straight up I can see over it.
The best position seems to be with it raised about 90% to allow me to look over it, but still gives me that nice pocket of calm air. Definately a great summer shield for long rides. Way better than stock and almost looks stock.
If you are worried about the top edge being in your field of vision than I would say go bigger, but if you just want the pocket of calm air with the wind going over your head, then the medium 2.5 should be fine if your 5' 10'', but remember everyone is different so go with your gut.
FYI - the medium Cal Sci for the 06 - 08 models does not have the extra cut vents in the lower part, however the shield is a solid mount and I noticed no real buffetting. Good luck.
