I'd put at least 6' (8 would be better) on the stick.
Another thing that is very handy (almost required) is an on/off switch for the thing. It's a pain trying to plug in and unplug the thing to turn it off and on. I built an extension cord with an outlet box on the end. In the outlet box is a switch/plug combo (top half of outlet is a switch, bottom is an outlet, $6). It helps a lot.
The switch is not waterproof, so there's a slight risk there - mostly from the spaz/freakout actions you might perform if you got shocked - because the GFCI should (should) keep it from actually killing you (you're using a GFCI, right?) But then if a non waterproof switch freaks you out, then a heatstick may not be for you anyway.
You could build the box with one of those outdoor, water resistant switch covers as well.
+1 on the 12AWG! I wouldn't sweat a 25' extension, as long as it's 12AWG (they're spendy but will save your tools)