I may well be wrong, but I always thought that before the Road Traffic Act of 1930, characters on the rear plate were supposed to be the same height as the front: 1 3/4″. The 1930 act decreed 2 1/2″ characters for motorcycle rear number plates whilst 1 3/4″ we’re still applicable for fronts. Cars were required to display 3″ characters. As I say I may well be mistaken.
I usually apply 1 3/4″ characters to rear plates of vintage machines because the proportions suit them better & so far I’ve got away with it!
Best wishes to all