Automotive off road lights wiring?

To help clean things up and make wiring a little more simpler on my jeep i got a fuse/relay box off an old junk car. i want to wire my off road lights into this relay box but i'm not sure what size wire is in the box it appears to be either 18 or 16 AWG one place said to strip the wire and measure the wire another says measure in the insulation. i'm not sure what to do. but if it is 18 awg would it hurt to go to a bigger size after the fuse box to my lights or should i rewire the whole box?