I'm 5' 11" and wanted to see over the top of the shield so I went with the 15" with their mounting hardware. The top of the windshield is about even with my nose and most of the buffeting is above my face shield (full face helmet). Overall, I'm happy with the shield. If someone local to you has a Gustafsson shield I would try to see if you could meet them and actually see it on a bike.
I am a new Valk owner (and new to this forum) and was posed with this same question. Being an avid Winger, I am a bit of a wuss when it comes to wind and head buffeting and decided to start with the 20 incher with their brackets.
By using their brackets, I can create a myriad of positions that allow me to either look through or over depending on what I want.
I am 5'9" and set the screen so the arms are pointed more horizontal creating a more upright screen and thus I have to look through it. But my ears are happy!
I did try it in lower settings and can create a combination that I can see over the top but with more turbulance.
See my attached pictures for clarification.
Sounds like the 20-inch model might be best for my height... I plan on purchasing one mounting bracket and installing the small 8-inch screen in dark smoke (I've got the black bike) for everyday use and then swapping out to the larger screen for touring trips.