These buggers are found in some of the smaller vibes out there. They are smaller than a AAA but bigger than cell batteries for a longer life span.
“An N battery (or N cell) is a standard size of dry cell battery. An N battery is cylindrical with electrical contacts on each end; the positive end has a nub or bump on the top. The battery has a length of 30.2 mm and a diameter of 12.0 mm, and is approximately three-fifths the length of a AA battery.”