Okay here it is:
grille exterior outside dimensions (to the outside of the metal grille):
9 1/8" wide x 22" high
One the inside it measures 7 1/4" wide x 20" high, but then there are some 1/4" bumps that jut out from the top and each side that would also need to be considered if you're cutting a new hole.
Here's a picture of the backside of mine (it is in a fg mounting flange)