Harley windsplitter windshield
THX
In your case, I would guess you'd be good with the 14 inch. Of course this is all just a wild guess. As far as the 18 inch, the Heritage has that stock. I know, not the same bike but it would give you an idea. At a minimum, you need to be able to look over the top of the windshield.
That being said, what I found got rid of buffeting was a set of lowers from Memphis Shades.
After having the windshield for some time, I finally figured out the wind causing the buffeting was coming from under the windshield.







