There are many older threads about windshield replacement with many opinions since the questions of which shield is better or which one should I buy vary on you needs and tastes.
When you see a certain length shield as working best for a person you need to know things like do they ride sitting upright or slumped, do they ride with no helmet,half helmet or a full face one etc.
As stated above and in the old threads you will see people stating their height and inseam measurement. The reason for that is you can get an idea on whether the rider is long torso or short torso since you can have two people same height but if it's due to long legs they may ride shorter then you if your height comes from a tall torso. That would change how the wind hits a rider with the same height windshield.
The most popular companies mentioned on this site tend to be Clearview Shields, Freedom,and a few others I can't remember now......many of them allow you to try a shield on your bike with the paper left on to see if it is good for you and if not you can return it for a replacement if not damaged. The Clearview Shield website has the instructions for measuring to help you select the right size for you and your bike.
Some of the companies have offered a site membership discount in the past.
Good luck in finding the right shield for you.