I typically use the following;
1/ Dynamo/Alternator and battery feeds along with main earth wires 2.5mm Sq rated at 30A continuous.
2/ All other wiring 24/0.2mm which is rated at 7A continuous.
One point to note is that I recomend running an addition earth wire up into the head light shell, and then having 1 common point near to the battery and charging medium
I also use various colours to help with identifying faults or problems at a later date, and also I use high quality waterproof connectors
Best of luck
Just being old fashioned I would be worried about using 0.2mm wire anywhere due to its lack of flexibility, lack of substantial sheath plus suitable safety margin, cos I'm scared of derating I dont think I would use anything less than .75mm. (not a criticism cos Im over cautious & ride like an old lady anyway!)
All the cable I spec is tri-rated 240 VAC rated cable, so is more than capable of handling anything either 6 or 12V DC can throw at it, including 60/55W headlight bulbs
cable I get from R.S. is quoted as 14A @240 AC, so I suppose low voltage DC will handle less current? I'm not sure as I tend to always over engineer!