1/2" drive is the size of the square post of the wrench; 1/2" x 1/2". 21mm socket, which has a matching 1/2" x 1/2" drive hole will fit the wrench. Drive size has nothing really to do with the amount of torque that can be applied, however you don't want a 3/8" drive (i.e., smaller than 1/2") if you can help it, as generally it will mean the wrench will have a smaller maximum torque that can be applied. An example:
edit: As it appears you're not familiar with a torque wrench: A torque wrench generally has a twistable handle which is calibrated with the amount of torque that will be applied before the wrench "slips" so that no further torque can be applied. Said torque is measured in foot-pounds (ft/lb or ft. lb.) and manufacturers have established the amount of torque required to keep nuts / bolts from loosening.
edit2: Sorry, it looks like I'm wrong about the torque value used for the lug nuts. Tesla spec IS in lb-ft. (a vector force) and is slightly different than my understanding of how torque is typically measured / stated. My experience is that torque wrenches are expressed in ft-lb (amount of work). I believe that there is a conversion factor that can be used to change one to another but if I understand tools correctly, as long as you keep your hands in the area of the torque wrench where they're supposed to be (i.e, the end and not the middle), applying the force needed to make the wrench slip (click) will be correct.
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