I wont get into another protracted "discussion" about it .. but for those who really want to know AND understand the law on number plate size it is quite complicated, especially when people start throwing about links to various legislation that might be relevant in it's own right but not always correct for particular circumstances...
You need to understand that Construction and Use regulations, i.e. those that govern the required standard of motor vehicle construction and condition for road use, are complicated and dont align properly with other legislation like MOT and specific amendments or other legislation for number plates.
You might pass an MOT but then still not be entirely legal for road use in the UK...
You might purchase a new vehicle that was made in the EU or made outside the EU then "homologated" by the imported/dealer for road use in the UK but still find some technical detail might land you with a fine or endorsement.
When you add into the mix the simple fact that today’s police are not as well trained or experienced as they used to be then it really does become a fuggs muddle.
Various conflicting advice from the official police guidelines emanating from ACPO together with conflicting MOT test standards and complicated and often badly worded legislation that is further altered by EU legislation means it's almost impossible for the police to keep up anyway.
Budget cuts for police and change in policy mean that where we once had a dedicated and professional police road traffic department where officers would stay for the remainder of their service, gaining knowledge and experience, we now have an interchange system where officers stay for several years then get rotated out of department...and now the system is changing again..
So you could well have a "legal" number plate on the back of your non homologated made or adapted for primary off road use trail bike but get stopped by a police officer who does not fully understand the rules...
If you look hard enough there is an allowance for a plate with the correct size numbers and letters for motorcycles to have smaller spacing and boarders.
You don’t have to have a rigid backing plate and you don’t have to have the makers name or a kite mark displayed on the face of the plate.
But you do have to know how your bike became to be registered in the UK and if it was homologated or subject to a single vehicle examination test.
There is also an ACPO guideline (2008) about any plate needing to be legible at a certain distance and for the police officer examining it to believe that there is no intention to avoid ANPR detection. This guidance is still relevant but was due to change last year..and as with all road traffic legislation it changes often and quickly.
Again it's very complicated so unless you are willing to go head to head and argue with a police officer and suffer the consequences if you get it wrong then the best bet is to follow the basic guidelines for motorcycles and not to try and be a smart arse.
I do have a full size normal plate for my bike and a smaller but still legal flexi plate for use that will have an "off road" element.. this is also a factor that needs to be considered. As UK trail riders we don’t go "OFF ROAD" normally so the "off road" element pertaining to the allowance for a smaller plate won’t apply. But when I ride locally I do go "off road" on permissive and private routes.. it's very important to be able to demonstrate that if asked
My smaller plate is correctly lit by the rear light and remains at a suitable angle to the vertical to remain visible. It displays the correct font and size of letters and numbers and has colour matching bolts (yellow on yellow and black on black).. It is not obscured by a stick on reflector that now also needs to be rectangular. The makers name is on the reverse side edge so is visible form the front of the bike along with a kite mark stamp.
But you can also get full size bendy plates... The idea is to look legal and avoid getting stopped...