0.75 mm2 is good enough between the fusebox and the relays, they don't draw much current. You could take the current from the dynamo or the starter, depending on where you want to place the relays. I would use at least 2 mm2 wire and keep the combined length, source -> relay -> head lights, as short as possible. Both are good for minimizing the voltage drop. Don't forget to put a fuse in this circuit, 10A should be good for two 55W bulbs.