With a sheet off of a small size notepad, a hole in the center that is small enough to pass the extended nut design, press the paper around the flats of the nut so you have an indent of the face of the nut size. Take a ruler, tape measure, and find the flat to flat at the half point of the indented paper. If both inch and mm are on each side of the tape measure, find said mm size and there you have it. That or take the inch and convert the number to mm online so you can see the socket size needed.
Chrome finished sockets are too brittle and may crack during impact be it breaker bar or air gun. Ass me how I know.
Black type looking sockets are more suited to heavy air gun work. Follow a tool truck to its next stop, enter truck and buy impact(heat treated = black finish) sockets, not chrome for this kind of work.
Pass on the bonus round... 
Tormenting the motorcycling community one post at a time