12 ga. solid core wire is preferred because it's slightly bigger and conducts heat more efficiently; but 14 ga., as shown here, is OK.
Make the drain heater by wrapping one end of the wire around the heater a few turn, as shown here. The other ends will be fanned out in the drip pan and run into the condensate drain hole two or three inched. Just watch out of the sharp fins on that evaporator, shred your hands like ground beef before you even knew what happened. Ok, it's not that bad, I just wanted to talk like a crusty old timer. Humor an old man.