You want a cord that's long enough to reach the work area and big enough to handle the wattage you need.
John Drengenberg with Underwriters Labs explains why it's not a good idea to plug several shorter cords together.
"If you daisy-chain extension cords from one to another, that means that there are a number of connectors running to the outlet from where you're using the tool," he says.
"The fact is, it's very easy to drag one of those connectors through water and that could be a shock hazard for you or for anyone in the area. So try to keep the cord length the proper length for where you're going to plug it in to where you're going to use the tool."
Drengenberg reminds us that we should never use an indoor extension cord, one of those smaller lamp cords, for outdoor projects.
"Those are for indoor use only," he says. "Make sure it's the larger, thicker cords that have better insulation and are a little bit more rugged for outdoor use."
UL: Extension Cord Safety Tips