Well, here's one answer to my own question.  I did a bit of poking around on the Internet and
found this site.

Bill
----- Original Message -----
From: "Bill N" <freeholder@STARBAND.NET>

I was sitting in my Westy, wrapping Christmas presents, many of which needed batteries, when I
noticed something.
Some of the items took D batteries, and some took C batteries.  Some took AA batteries, and
some took AAA batteries.
This lettering system implies the existence of size A and size B batteries.  Why have I never
seen one of those?  I was just wondering...