Crutchfield recommends 16ga up to 80ft, and they're in the business of trying to get you to spend money with them.
http://www.crutchfieldadvisor.com/learningcenter/home/speakers_wire.html

They also say "For audiophile-quality music systems — using the best wire possible helps bring out fine musical detail". At least they didn't attempt to specify what makes a wire the "best possible" or else I wouldn't steer anyone to this website.