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If you are talking about the rear license plate, it should be 10mm hex bolts.
I bought hex bolts for my rear plate yesterday and 6mm (diameter) hex bolts fit my holes. The shortest they had in stock was 20mm but I could have gotten by with less length. I had to crank them in and bring them back out to clear the plastice from the threads as they were binding a little going in.
A word of warning! Using bolts to long (24mm) will cause the panel surrounding the license plate to begin to bow out from the trunk deck!