VaBob - Happy to help you with any info I might have on deciding on height. It does depend on which saddle you're using, your height, inseam (which gives an idea of the ratio of your upper body length) and of course your personal preference on where the shield lives in your field of view.

I think I mentioned it in another thread on these shields here in the site, but I'm on the stock saddle (with a beaded cover), am 6'1" and 32 inch inseam...so I'm a bit long in the torso. That said, the manufacturer site recommended the 15" shield for 6'1" to 6'3", and that's what I bought. Works great. At full up, there's almost no wind at all. Full down, it's breezy but no buffeting.

Cheers!