Vehicle wiring products give a sizing guide depending on ampage in the cable.
https://www.vehiclewiringproducts.co.uk/c-48-cables/c-198-single-core-pvc-thin-wall-cable
I have just gotta say just received Chris’s wiring diagram drawing for my bike, it is a wonderful product and it will help me so much in the making of the wiring harnesses. It is clear, exceptionally well produced and laminated for my grubby oily hands.
I highly recommend anyone who is playing with their wiring to buy one of these.
It has made all your kind advice become a lot clearer.
Cheers
Ian