Ed's correct about not grounding to the building's frame. That's just not good practice. As far as gauge goes, you are looking for >1/4 ohm from end to end of your ground cable, which simply means the longer the run the larger the gauge. 4 gauge copper has a resistance of .2533 ohms per 1000 feet but 6 gauge is .4023 ohms per 1000, so you could only go about 625 feet before you exceeded your 1/4 ohm limit.
