Wattage divide by 13.8 volts = amps

example:
50w lights x 2 since there are 2 of them = 100w

100 divided by 13.8 = 7.24 amps

typical fog light wiring is about 18-20 feet

so you would need... a 12 ga wire

You want 12 because you should ONLY RUN 80% of a circuits capacity

Take your amps and go to this chart for the length of wire and what gauge
http://www.gtsparkplugs.com/WireSizeCalc.html
Also, make sure your switch is rated for the amps, if not you will need a relay