You couldn't be more right, dumb question indeed. For what it's worth Home Depot has this really neat rack of screws and screwholes and you can take any screw there to figure out what size (english/metric) and thread (coarse/fine).

Turns out the new license plate holders use M5-.8 (5mm-8), I got 12mm length should be just about right since the ones for my older plate holder were just tad too short and this screw was just a little longer. A bargain for 2 for .78 cents!!!