How do two DC motors of the same power but for different voltages differ in size?
If you have a motor at voltage V1 and you want to have the same performance at a new voltage V2, then you have to multiply the number of turns by V2/V1 and you have to multiply your magnet wire CMA by V1/V2.  So, to go from 18 V to 36 volts, you would double the turns in your motor and half the CMA (circular mil area) of your magnet wire (that is, increase the wire gauge by 3).  

That is a rule of thumb.  When you go to do the actual ratings tests for the 2 motors, you will see some differences.  Some of it is due to more current going through lower voltage motors than higher voltage motors.  Some of that is due to the fact that larger wires don't bend as easily and don't lay down as nicely as smaller wires.  So motor manufacturers may tweak the number of turns or wire size to get the performance at the different voltages as close as possible.  

Doubling the number of turns and halving the wire gauge (or vice versa) ends up giving you about the same slot fill if you use the same lamination.  Smaller wire will pack better than larger wire so your effective slot fill will be smaller but overall its usually close enough that a motor manufacturer will use the same lamination.  

Here's a little more explanation of why you change the number of turns and CMA of the magnet wire.  You are trying to keep the flux density in the magnetic circuit of your motor the same. If you double the voltage, then based on the resistance of your coil, your current will be cut in half. If current is cut in half, then you need twice as many turns to get the same flux density. However, if you double the turns, then your resistance increases, so you need to decrease the CMA of your magnet wire by half to get the resistance (and thus current) back to where it was.