From ACU handbook....
5.8 NUMBER PLATES. They must be rectangular shape and be made from rigid and solid material with minimum measurements 285mm by 235mm
...even the bad grammar is per ACU book, nice!
Basically you need a plate big enough to fit your race numbers on. As long as the numbers are clear, you should be OK. The marshalls, race control and the timekeepers (if there are any) need to see the numbers as you pass at 100mph from 50 yards!!