The only thing I know is that the button frames should be 96wide x 64high.  When I was playing with it, it seemed like the real key was a ratio of 1.5w x 1.0 tall - since it gets resized sometimes I could a sharper image (on my machine) by applying the ratio to a smaller image such as 48w x 32h or 54w x 36h.  None of that changes the size the image displays though, as long as the ratio is maintained.  If the ratio is off, such as 64x64, I believe the button will appear too small if I remember right.

Dave, thank you for your input. I found a way to make it bigger. It's not perfect, but better than it was, and it is acceptable now. The secret was to trim the empty spaces a tad between the button elements and the margin of the file, a space which contained transparency. All  I had to do was just tinker with it a little.