What about Power Functions motor, S size? 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?
Only in 2012 the L-motor was released. The M and XL motors where released in 2007. That is a gap of 5 years where people were even more confused about the size options...

Anyway, no smaller motor with power functions

However if you are willing to go with the 9V system (possibly with conversion cables), there's the mini and micro motor, standard motor is also a bit smaller in length.

More info here:
http://www.philohome.com/motors/motorcomp.htm