The law does not specify what size the plate is.
It actually specifies what size and font the characters should be, the space between the characters, space between the lines and the size of the border around the characters.
The characters on a motorbike should be 64mm high and 44mm wide. There should be 10mm between characters and also an 11mm border around the entire plate.
The plate must be on two lines.
So the size of your plate depends on the number of characters on the plate.
This information is taken from the DVLA.
Hope this clears things up.
http://www.dvla.gov.uk/media/pdf/lea...mberplates.pdf