On Mississippi car tags there is a space in the middle between the numbers, and lots of people put decals between them. The measurement Is 3in x 3in. It might be to small to bother with, because every States tags might not be the same as ours. Not a big issue I would order one of the bigger ones, and put on my back glass if you didn't have some small ones made.
The back window of my truck is dark tinted - My vote would be fo an 8-10 inch white diecut. I don't recall the name of the material but it has a white image on a clear liner - you apply the decal to the window and peel away the liner leaving the diecut image on the window.