My electrical box has 14 awg wires and I have a 12 awg copper ground wire lying around. Can I use the 12 awg copper or do I need to get a 14 gauge?
Should be fine. If it's just pigtails in the box it won't even affect your fill, IIRC. If you are running 12Ga ground wire (not just in-box pigtails) you do have to allow for it in box volume calculations, but it would rarely cause a problem since the equipment grounding conductor is only counted once for box fill.