2 4s lipos? the fattest bullet connectors you can find, and the XL2 uses 8 guage wire, if you're using 2 batteries, you can use 10 guage wire, though i would recommend 8
or if you're using one 8s lipo, use 8 guage cable
the main thing is, if you go higher than 8 guage, its pointless because the esc uses 8 guage
Jay