Just bought a new rear number plate and it is about 4 3/4" wide. The existing front one is about 5 1/4" wide. Does anyone know what size they should be?

Department of Transport has it all on their web site, but it takes a bit of finding if you are an old fogey like I am! Basically old fashioned plates can have characters 89mm high with minimum 13mm space above and below. I did not see it today, but they used to say that the modern font was not required so long as the characters are easily readable.

The original pressed backing plate is 4 3/4"x20 3/4"

Assuming that Ian's number plate mentioned has a swage around the periphery, the "flat" area is about 3 3/4" by 19 1/2". But when we say "original" - Stanley Edge's measurements were 19 1/2" long by 5 1/8" high (no swage) with letters/figures 3 1/2" high by 2 1/2" wide. The Chummy version had the Z-shaped tail-lamp bracket rivetted on behind the RH end, plus a wide curved bracket rivetted behind top centre for mounting over the sparewheel onto the carrier, whereas the Sports as well as the tail-lamp bracket had two angled brackets (rivetted behind top centre) with slotted holes for quick removal. Cheers, Bill in Oz

Yes Bill my plate has the swage around the edge and is 3 9/16” x19 ¾” on the flat. These swaged plates were used as backing plates right through the 50s and 60s by BMC/ British Leyland on most of the range from Austin Cambridge, vans and to the front of early Minis finished in stoved black. The usual thing was to fit a pressed numeral plate bolted to the backing plate. This backing plate was still used on BMC/ BL Commercial vehicle chassis that were sent to the Coachbuilders for body fitting well into the 1970s along with a temporary set of wings made from two pieces of 2”by 2” and hardboard!
The advent of the yellow reflective plate around 1972 seemed to be the end of the separate backing plate here in Britain. I am sure the ones on both my cars are from that era and pinched from work! ( I cut out and riveted the numerals myself from 1/8” alloy plate to save money at the time!) As far as I know the original 1920s plates are only a 1/16” or so different from the above. To all intents the same.

I realised that there was some information about numberplate and letter sizes in Ghris Goulds book "A Guide to Building Reproduction Austin Ulsters".Unfortunately the information is slightly contradictory as one view gives 20 3/4 X 4 3/4 whilst the other view, complete with letter & spacing information, gives 18 1/2 X 5 (all in proper measurements of course)