Any windshield needs to not be higner than the bridge of your nose. On long rides your posture slumps and what seemed okay for a few hours may not work. So, as long as it is never higher than the sight of vision, anything less is good, but way less may be in style, but the shortys will work your neck as the wind hammers you on those all day rides....

Hogback

 

http://www.clearviewshields.com/
http://www.fastaire.com/
http://www.eglidegoodies.com/