You can look at all the different sizes of shot on Federals web site
http://www.federalcartridge.com/
Look in the Ammunition catalog, under classic lead shot.
According to the Encyclopaedia Britannica;
"The sizes of shot used for small game hunting vary from about 0.08" to 0.15" (2mm-4mm) in diameter, and are identified by arbitrary number systems that vary from country to country."
Then it goes on further about buckshot sizes.
Chuck
[This message has been edited by chucko (edited September 25, 1999).]