I'm told the top of the screen should be level with your bottom lip. (Don't know what happens if you open your mouth...). And that you need to be able to see over the top for visability in the wet etc. All this gleaned from the forum because I've never used one but a bit tempted to try. 

With the FL fatboy frame a "Memphis Shades" brand quick detachable could be the go in "Fat or "Slim" version. the Fat looks good if you have a lightbar in place but may be too wide for the standard Fatboy look.

http://www.memphisshades.com/harley-shades/memphis-fats-a-slim

You can get a detachable mount for it with a not-so-obvious clip attached to the sides of your upper forks about the height of the hedlight tins and goes on or off in a minute or less I had one of htese when my FLSTN was in OEM configuration and was very happy with it.

lAs Taffy mentioned best to size it to your lip height sitting normally in the seat.