I think the trapezoid shape was used in the Cheyenne tail turret. I was actually not used as a window but was mounted in a frame attached to the guns inside the turret. It was located between the operators head and the reflector gunsight and served as a shield to his head. I "think" it only moved side to side when he moved the guns.
https://ww2aircraft.net/forum/thread...a-b-17g.53737/
https://www.flickr.com/photos/kensaviation/2487953543
https://paintingvalley.com/download-...-drawing-2.png
As to the size, I have not found that yet. The size may have varied over the years. I believe the glass had a greenish tinge.
This video has a lot of info about the evolution of the tail turrets. The man who made the video rebuilt a Cheyenne tail turret, getting parts from many sources. It may be possible for you to contact him and actually get the measurements of the glass.
https://www.youtube.com/watch?v=_9j98R3_rbg