Just Googled road legal plates & found this:
All motorcycles and tricycles must display a number plate at the
rear of the vehicle. Motorcycles and tricycles registered before 1st September 2001
may display a number plate at the front of the vehicle but are not required to.
If you have a two or three wheeled motorcycle or tricycle that has a body type of a four-wheeled vehicle, you MUST display a number plate at the front
and back of the vehicle.
Motorcycles registered on or after 1st September 2001 must display a two-line number plate. Motorcycles registered before 1st September 2001 may display a three-line number plate.
One-line number plates are
illegal, regardless of the date of registration.
Here are the character sizes:
| Character Height[/t][/t][/t] |
| 64 mm |
| Character Width (except the figure 1 or letter I) | 44 mm |
| Character stroke | 10 mm |
| Space between characters | 10 mm |
| Space between groups | 30 mm |
| Top, bottom, and side margins (minimum) | 11 mm |
| Space between vertical lines | 13 mm |
so i suppose you can work out that a box standard plate AB 12 ABC would have to be 9" x 5"[/td][/tr][/table]