Okay, here are some examples of what the stickers would look like at various sizes (8, 10, and 12 inches long). My back window is 46 in. - scale is approximate, but pretty close.
Hmmm, maybe 9 in. would be a good compromise between the size of 8 in. and the legibility of 10 in.
EDIT: removed the pics from my site because my allowed bandwidth is really low...