When we were installing "drain" wires (be careful calling them "grounds" or you might get into 250 trouble) our enginerers said a 16ga was plenty. It all depends on exactly what you think you are bleeding off. If it is really high frequency "noise" you want a very fine stranded or braided wire. You probably get more surface area in a 16ga fine strand wire than in a regular 2ga "stranded" THHN.
When this actually fixed something (notably a noise problem on long printer cables) 16ga stranded THHN did the trick.
