Update: The fender skirts that I have are 37 skirts. The flange on the back of the skirts is larger in diameter than the wheel opening on 38-39 (and 40) rear fenders. The flange is the same size as the wheel opening on 37 rear fenders.
So, for correct Ford fender skirts:
31 inches "pointy bit to pointy bit" by 18 inches tall = 37 fender skirts
29 1/2 inches "pointy bit to pointy bit" by 17 inches tall = 38-39 fender skirts.
To confuse the issue a bit I recently purchased what I was told are very early after-market fender skirts. From the outside they sure look like Ford skirts. The back looks a bit different. The really interesting thing is they are the same size as 37 skirts, but the back is smaller, so the skirts fit 38-39 fenders. Here is a comparison of the skirts:
One of the after-market skirts in front of a 37 skirt. They are the same size:
The flange on the back of the 37 skirt is right out at the edge of the skirt. This matches the configuration of the 38-39 skirt that fordlovr posted the picture of:
The back of the after-market skirt looks very different. The flange area is a series of straight sections, and they are nowhere near the edge of the skirt:
The after-market skirts look good though, so I will eventually run them on my 38 coupe.