I'm thinking more on the lines of 3" x 1". Here's my reasoning:
1) Anything less than 10" x 3" is NOT "readable" at normal driving distances (proof -- the handwritten info on store-bought "for sale" signs are hard to read).
2) Most MBZ owners are "overtly discreet" -- we like to brag without really bragging.
3) If your cool MBZ is in a parking lot and people are checkin' it out, they'll see the decal at that point.

That's Brian's 2-cents