Optimal size for a windscreen depends simply on rider hight and character of riding. Touring enduros have quite tall frames which leads to riders from the taller end of spectrum, but several enduros can be tuned down with accessory seats and by changing the rear suspension bottom link. Therefore the rider hight spectrum is wider than ever. Generally there are two size of standard windscreens. Models that are mainly designed for urban and every day riding have usually shorter windshields which provide a minimum coverage. Models that are mainly designed for grand touring provide more coverage also by the windscreen. There are usually even more adjustments for the windscreen, perhaps also spoilers. Read more here
