Actually, there are B batteries. Source: Answerbag
"There is a 'B' size battery, but it is not used anymore. A number of battery sizes were standardized by ANSI (American National Standards Institute) in the early 20th century. Although a number of battery sizes were defined, industry has generally used fewer and fewer different designs over the years. When was the last time you bought an 'N' battery, for example? This helps reduce costs for the manufacturers, the retailers, and the end users. 'B' batteries only collect dust on the shelf today, along with the 'A', 'E', and many other battery sizes."