LEGO Power Functions has motors in M, L and XL sizes. Usually, if someone uses this size designations, the smallest one is S. It would make sense to have them named S, M and L if there were meant to be only 3.
Now, for my personal "rebuild, restore" project, I'd need something a bit smaller than M, to go with my old sets. Is there any S size power function motor? Was there? Was planned, but cancelled? Any official or semi-official info on it?