I actually just measured myself and am printing out my first custom inserts tomorrow.
The dimensions for the label for each key are 7/16" H x 8/16" V, except for numpad Enter, which is 20/16" V.
In MS Paint, 1" = 72 pixels, which means that each label should be about 36x40 pixels, except for Enter which is 36x60.
I can email you the .bmp with my icons if you want an example. I'm going to print them out tomorrow, cut, and pray that they fit right.