I gotta agree with Mac, I have a Stremliner Spitfire and find it is a little small but the difference with and without it is night and day.  I live in the country so most of my riding starts down the hwy at speed of 90 - 10 KPH (55 - 62 MPH) and with broad shoulders I felt that after I got over 80 (50) that I was just hanging on with fingertips.  It is also very windy here which plays a big factor.

Take into consideration your height and riding position to make sure you don't end up with a windshield that redirects everything smack into your face or that blocks your vision because it is too tall.  I wear a full face helmet so it isn't a huge deal that mine is a little too short, it takes the pressure off my chest which is what I wanted.