My understanding is that a size 0 marble would fit through a 5/8" hole but not a 9/16" (00) hole. So a size 0 marble could range from 9/16"+ to 5/8". Likewise, a 00 marble could range from 1/2"+ to 9/16". In essence, I think marbles were sized based on the smallest size hole they would fit through. A true peewee would have to fit through the 1/2" (000) hole. And I've never heard of a size 0000, which would be a 7/16" hole, or smaller. Although, I definitely have seen marbles smaller. FWIW.