The rear hatch glass is 50" wide (at the bottom) by 19.75" tall (in the center). However only the bottom of it appears straight. The top has a subtle curve in it down towards each end, the sides taper towards the top and each corner is curved angling in towards the top.
If you happen to need one, I have one n the Classifieds, out of a 1997 80 with factory tint and a good defrost grid.