I am a little premature, but I am painting my car this weekend and need something more to look forward too. I heard 8" #s are what SCCA requires.

here you go, they should look something like this....make sure they are contrasting to the color of your car....you want the corner workers to know who you are and where you are.....numbers that look good from 10' away, look like crap at 50'.

from the PCA-GGR rule book

4.6T Driver's Education/Time Trial
a. Car numbers (both) and competition class letters (TT‐only) must be on the car and correct for the driver at all times in order for course personnel, event officials and your fellow competitors to identify cars and their drivers. Numbers must be a contrasting color, and distinct from a distance. Reflecting numbers are discouraged and shoe polish is not allowed. No numbers on windows. Numbers must be from 8 to 10 inches high and 1‐1/2 to 2 inches wide and must be securely attached. The number on the rear may be only half the regular size but still legible. The car classification letters should be half the size of the competition number and placed after the number on both sides of the car.
b. Numbers shall be placed on both sides of the car, on the front and the rear of the car.
c. Location, style, legibility and mounting of all car numbers are subject to approval of the Tech Inspector.
d. The driver number is your GGR Certification Number. Drivers sharing a car may use the same car number but with the suffix 'L' attached to the number in one case to distinguish the drivers.
e. Car numbers will remain as designated for a period of one series year. Nonparticipating drivers may reserve their number for $25.00 per year for a maximum of two consecutive years.
4.7

Thanks Dan.

A long time ago I was told that "large numbers mean you are serious" or that you want to make sure that the timing folks AND the announcer can read them from where they might be. Be sure to go with lots of contrast such as orange numbers on blue paint, etc

I like the size of Dan's numbers personally... makes things easy for all trailer reading talent..

I went with a 16" circle and two 8" numbers inside. Could not really fit anything much larger on the door. If you're just putting a number, then you can go much larger, but I liked the old school look.

For plain numbers (no circle), I think 10" or even 12" looks better than 8". The smaller numbers seem 'unbalanced' on a large expanse of door or lid sheet metal IMO.

POC rules are a bit different from PCA but I went with 10" and 4" sizing.