I remember tearing apart a 6 v lantern battery years ago, and it had 4 round cells in it, they weren't any standard size.
The voltage generated by a cell depends on the chemistry. With the zinc-carbon stuff, you get 1.5 volts, 1.2 v with nicad, 2 volts with lead-acid, etc. Any battery putting out more is then just a combination of these in series (or perhaps series and parallel in the case you mention). Note that a car battery has 6 acid caps- 6 cells @ 2 volts to get the 12 total.