I am on my laptop so I cannot give you exactly until I get to the shop tomorrow and look at my patterns but what I have on this computer is the length is 25 1/2 inches long. The front is 19" (+/-) tall, and the rear is 19 7/8" (+/-) tall. The original window (and the repro window channels) are based around the original thickness of 3/16". If you need something before I get back to you, cut out a pattern with that measurement and see how it fits.
Classic Flat Glass has it listed as 16X26 inches for a 31 Ford Standard Coupe but they don't list the corner radius or other particulars that would have been on the pattern. It seems like one of the model A parts vendors like Snyders, Brattons, or Mike's A-Fordable had dimentions in there catalogs but I can't remember for sure.
There is no corner radius, it's square, also keep the glass stock height, even if the car is chopped, so you don't have to move the stop.
The over all shape is rectangular but the corners are 90 degrees, I wish I had my car here so I could give you exact dimensions.
Keeping them stock height will also keep them straight while rolling them up & down , unlike my coop that gets crooked !