Shot size nomenclature is confusing --- in birdshot sizes, such as you mentioned, the larger the number, the smaller the diameter, so, #6 is smaller than #5, and so on, until #12, the smallest numbered size, followed by size "dust", which I've heard of, but never seen loaded in commercial shotshells. #12 is the tiny size used in .22 rimfire shotshells, for example. Buckshot sizes on the other hand range range from 000 ("triple aught") upward thru 00, 0, etc. to #4, in descending diameter. There are additional designations for non-lead shot sizes, as well. Not every increment of size is commonly available or even manufactured as far as I'm aware. You can find loaded shells in #6, 7-1/2, and 9, but I don't think I've ever seen #8 or#10 or smaller in standard guage shotshells, for example. This is tech trivia fraught with arcane measuring and descriptive systems --- approach with caution!