a 10mm spark plug socket has a 16mm hex size. I believe that the 10mm relates to the diameter of the ceramic element of the plug as most plug sockets have a rubber insert that is just smaller then 10mm internal diameter so it grabs the socket to allow it to be removed from the plug 'hole' easily. Another useless fact is that a 16mm spark plug will have a 14mm thread with a metric 1.25mm pitch.
some tips: when removing the plug its always easier to do on a hot engine than a cold one. Re-installing the plug: push the plug into the socket and then into the trhread, turn counter clockwise by HAND until you feel a click this means that the 2 threads are aligned and will tighten in without cross threading. Tighten the plug by hand and then torque up tight, release the plug and then tighten with a torque wrench. This crushes the compression washer and then retightens it to the right torque meaning that the plugs wont be lower than the desired torque once they heat up.
Bored yet? I could go on.....