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.
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.