I'm 6' and the 17" Clearview (Summer Screen) is just tall enough for me. No doubt, the 18" was too tall. Be sure you are looking over the top at anything on the ground 50' or more from the bike.
I feel the best way to measure is to sit on the bike in riding position and lay a yardstick level across the top of the shield. Have someone else eyeball "level". It should hit between your mouth and the tip of your nose, ideally.
