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:
[table] [tr] [td] Character Height[/t][/t][/t]
[table][tr][td] 64 mm[/td][/tr] [tr] [td] Character Width (except the figure 1 or letter I) [/td] [td] 44 mm[/td][/tr] [tr] [td] Character stroke[/td] [td] 10 mm[/td][/tr] [tr] [td] Space between characters[/td] [td] 10 mm[/td][/tr] [tr] [td] Space between groups[/td] [td] 30 mm[/td][/tr] [tr] [td] Top, bottom, and side margins (minimum)[/td] [td] 11 mm[/td][/tr] [tr] [td] Space between vertical lines[/td] [td] 13 mm[/td][/tr][/table]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]