Glimmie is right. Bigger than needed is not harmful. If you want to read more on this topic:
If you do not want to have to change wires if you change to speakers with a lower impedance, going a bit big can be practical. As noted at the link above, this is particularly relevant when installing wires in a wall, as changing them to something else would be troublesome.
With that in mind, I normally just buy 12 gauge these days. But it is larger than it needs to be in all but one of my uses of it. I still have and use some 16 gauge wire, which is commonly large enough.
Edited by Jack D Ripper - 2/3/14 at 9:02am