The starter comes with 16 ga wire and that is what I use on mine.
The longer length of the coiled cord will cause a voltage drop and IF you intend to use it on LARGER engines the heavier wire may be better suited.
I have not seen the 10 gauge speaker wire that was suggested but it would be a good bet. Better to err on the heavy side rather than too light. We have used 10 and 12 gauge coiled cords on various industrial designs so it is available. Most likely a coiled type SJO cord.
I surely wouldn't use anything less than 16 ga for normal use and 18 would be marginal even for a 40 size two stroke.Especially in a coiled cord.