I would think 12 gauge should be fine since that's what is used for household wiring. My only concern would be the distance, so maybe bumping up to 10 gauge would be a good idea to be on the safe side.
But... the price doubles with those specs:
http://www.harborfreight.com/50-ft-10-gauge-triple-tap-extension-cord-93670.htmlLike I said, the 12 gauge will probably be fine, but I tend to err on the side of caution when it comes to stuff like this where electricity and a possible fire hazard are concerned.
Maybe there's an electrician here who can answer this for you more accurately.