Welcome to the Forum, and congratulations on your new rig!
(My Missus described this process to me...
)
If you put a tape measure at the bottom rear edge of one of the driving compartment windows: just work your way around to the other side of your rig.
Note that measurement, and then just measure rearward from that point, to a good "anchoring spot" in the Coach. (Remember to add in the same amount for the other side...)
Now you know the total length of the curtain...
Just add velcro to the top edge (e=just enough to keep it in place...), and you'll be good to go!