12 gauge is probably what you want, just to be safe. You could likely get away with 14, though. A nice 12V wire sizing table can be found here: http://www.affordable-solar.com/wire.charts.htm

For lower voltage, you need thicker wire, for higher voltage, you can get away with thinner wire. The chart says 10A can be done with 12awg at 12V for 7 feet with only a 2% loss. Halve that distance for 6V, leaving you 3.5ft.

You'll be charging a voltage somewhere in between, so 12ga over 3 ft should be more than adequate.

Note: I used 10A as the example because that's the max of your charger, which you should size for now, even if you're not using it. Then you won't have to make another harness again down the road. Wire is relatively cheap anyway.