Most of the legality of a number plate is the requirement that the letters and numbers have to be of a certain size and spacing.Quite rightly as you say you are not allowed to misrepresent a letter or number so that it looks like something else.
It also has to be easy and clear to read, therefore if the lettering is the correct size obviously the size of the plate has to be big enough to accept the lettering on it,however as you have rightly said it does not go into the actual size of the number plate.
There is one requirement and the extract below clearly is a British Standard that has to apply to the design of the plate, there is no mention again about the size. I hope this helps, by the way did you have a witness to this conversation because if this gets naughty for you it would be of great help.
Keep us posted
THE BRITISH STANDARD BS AU 145d
The British Standard sets out the physical characteristics on the number plate. This includes:
* Visibility
* Strength
* Reflectivity
The British Standard also requires that a number plate must be marked with the following information: -
* The British Standard Number (currently BS AU 145d)
* The name, trade mark, or other means of identification of the manufacturer or
component supplier. (The company who actually make the number plate.)
* The name and postcode of the supplying outlet. (The 'supplying outlet' is taken to
mean a company whose business consists wholly or partly of selling number
plates.)
* A non-reflective border and the Euro-symbol with the national identification
letters are optional additions.
* There shall be no other markings or material contained on the number plate.
Further information about the British Standards requirements can be obtained at BSI Group