Hello Vinnie
I looked at the Aero Detail 05 - Messerschmitt Bf 109G book and based on the drawings included in that reference it would appear that the windscreen was the same for the G-series. Unfortunately, I have no idea regarding the measurements of this but I did come across a replica for sale at
http://www.aeroart.de/en/detail.php?artikelID=64. Perhaps you can contact the sellers and ask for the dimensions?
Horrido!
Leo