The Straight Dope
: "The letters are part of a standard for single-cell batteries devised by the American National Standards Institute, or ANSI, beginning in the 1920s. (I realize that, strictly speaking, a battery consists of two or more cells, but let's not get picky.) Today the standard sizes range from AAAA to G, and for some reason there's also J, N, and 6. AAA, AA, C, and D were the only sizes that caught on in a big way commercially, but the others haven't totally disappeared. If you pry apart one of those big 6-volt lantern batteries, you'll find four F cells inside." (thanks, Brad)