The snap in retainer is 3/8" tall and 7/8" wide.
Image attached.
I'd love to know where I can find this stuff!
The window pane is roughly 33 X 15 so I'm guessing I need about 126 inches of the spline.


Thanks for posting. I would start by looking at our 55-204 snap-in glazing. We believe this is the one you are looking for. It's mostly used for awning windows. I recommend looking over our dimensions just to be sure they line up. Ours are just a little different from yours.
You may use either a heavy duty pair of scissors or snips.