I worked with Clearview on the replacement Summer Screen. The pictures of the Triumph America on their web site are mine. I'm 6' tall and the 17" replacement works fine for me. I might could use an 18", but it wasn't available when the first one was made. I'm definitely sticking with the 17" as I don't want a windshield any larger than required to do what I want - get the buffeted wind over my head. My feeling is that you would probably need to be taller than 6' to need the 18".
I've spent a great deal of time working on this. There are a lot of different recommendations on how to judge the height of the screen. I've found the very best way to measure is to lay a yardstick across the top of the shield that is level with the ground and see where it hits you seated on the bike. You probably need another person to sight where it's level. It ideally should hit you between the top of your mouth and the tip of your nose. If it's below that, you can go up, but if it's above, you are too high.
If you have a Summer Screen and are looking at a Clearview replacement, get the 17" if you are 5'8" to 6' and the 18" if you're taller than 6'. I would still check the measurement with the yardstick before removing the plastic on the screen. Like I said a person 6' might could go either way, not all bodies are built the same. The level yardstick measurment is the best judge. Drive the bike before removing the plastic and make sure it is ok. You can exchange the shield if the plastic has not been removed.
