Alberto,
You can get a good idea of what to order by sitting on the bike, in your normal riding position and having someone holding a ruler or tape measure on the top of the OEM screen you have (?) and then using that number of inches where you can just see over, rather than through, a new screen.
If you order one that turns out to be the wrong size, Clearview allows you to send it back as long as you have NOT removed the blue plastic covering they protect the screen with while shipping. Or, because sending it back might be expensive from where you live, it is quite easy to cut the screen down a bit with a coping saw blade and sand the cut edge lightly with a fine sandpaper block.