We actually have more sizes than those listed by Mike Campbell. There are also 11s and 12s I have seen - think it was used to take small birds for ornithologists and is also in some of the .22 long rifle bird shot. Then we import English and Italian shot sizes among others and being metric their 6s are different than US sizes which aren't all that uniform themselves.
But Mike has given a good summary of the commonly used sizes in lead shot. Steel and and some of the composition shot introduced some larger sizes like T shot which makes a great coyote killer in Hevi shot.
