gncc numbers plate size

doesn't matter it can be big or small. they don't go by the numbers on your bike. they also have colored backgrounds depending on class

I m helping to organize a race and I need suggested sizes for all the ATV's

I wouldn't go by the quad number due to people borrow other people quads put a number on the helmet and your good to go

As stated above GNCC does nothing with numbers on the bike. They do helmet numbers and transponders. The local series here does a sticker background with the color of your class and the classes have specific numbers. Like my class is white background and numbers 1-99. They also use transponders tho. The back ground sticker is 4x8ish and the numbers are a tad bigger than mailbox numbers.