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It is very important to correctly torgue them. Dont take lessons on how to use the torque wrench from the guerilla at the tire shop. The most important reason to torque them is because the rotors by disapating the heat into the atmosphere and through the rims. If you have one nut at 90, and the other at 100, the heat will pass to the rim more efficiently at the point with 100 ft/lbs, then the 90, causing the rotors to warp.
The size is 19mm, I'm told by a very good source, which is very close to 3/4".
However, in taking my lug nut to Sears, both the Craftsman 3/4" and 19mm fit the same (or seemingly so). I chose the 3/4" because it has 6 points vs. 12 for the 19mm.