Smallest legal number plate?

[Blou-Yster]

Does anyone know what the smallest, legal size of a bike's number plate may be.  My bike's plate is gigantic and it looks kind of stupid.

 

[Dr Dirt]

 

[Blou-Yster]

 

[Robski]

 

[IRISH]

 

[emiel]

 

[BigEd]

 

[Buzzlightyear]

 

[Martin-DR650]

 

[RobCrouch]

 

[Ruder]

 

[hondsekierie]

 

[lecap]

 

[Cane Rat]

 

[SteveKLE]

 

[Orangeswifty]

 

[Orangeswifty]

 

[TornadoF5]

 

[Stofadder]

 

[Marnus]