With the windscreen I'm using it's completely gone. I can stay with the visor up at 100 km/h. it does start again at 120 - 130, but not as bad as with the OE tall screen (that was causing me buffeting, as I'm taller). I did a lot of research before buying the screen, as I saw that it's most likely trial and error.
To get an idea on how large it should be for you, use a measuring tape from the bottom of the screen and extend the tape (on the screen's angle) all the way to the height of your nose while you're sitting on the bike. That should be your ideal windscreen height.