John, if you have the stock Roadster screen see if you can get a relative measurement. With the bike on level ground set a yardstick over the top of the windshield holding it level (may need another person to sight) and see where it hits you. It needs to be between your mouth and nose (ideally) to stop the buffeting and wind hitting your face. By measuring where it hits, you can figure about how many inches higher it would need to be.
Cleaview Shield's largest Roadster replacement is 20" (4" higher than stock) and probably better than the factory one. If that 4" gets above your mouth (yardstick level) it should do the trick. I don't know of one taller available.
