I found a 6 caricter reg. after reading the regs. It’s slightly over legal size at 179×166 7 1/8 x 6 5/8 ..caricters have to be on 2 lines and displayed to the rear of rear wheel for msva (mot) regs.
some info contained in.
vehicle-registration-numbers-and-number-plates-inf104.pdf
lots of other regs on reflective ness of background and contrast of caricters ,oh and back in the day I can remember being stood on the road side on a very wet day while an officer measured my plate with a ruller.
regards martin