Small.
I'm assuming the wires are SPEAKER TERMINAL connection wires, and NOT the POWER LEAD to the amp... but still small.
At least 14ga. in my opinion... but 12ga. is more suited for this purpose.
IF you are meaning AMP power lead... then you better get at least 8ga. I, too, have seen a car AND a van go up in flame due to SPEAKER WIRES (likely about 14, 16, or possibly 18ga.) be used on an AMP POWER lead! THAT will surely be a quick ticket to no wheels, or at least FRIED wheels.
(Personally, I've not heard of speaker wires that were a little small CAUSING or resulting in a fire myself... since they are LOW current compared to the power source.)
-crisp
