I think you want wire that can take at least one amp of current. Looking at the table that Bald Paul posted, you probably could get pretty small. But keep in mind that a really thin wire can break pretty easily, say for example if you forgot to unplug the hub when removing the wheel (I speak from experience), but there is a reason I used thicker wire. Years ago when wired bike computers were common, I had too many wired computers become dysfunctional when I snagged the wire on something and broke it. I did not want a repeat on my dyno systems so I used thicker wire than the wire that was provided by B&M.
If you are curious I documented my wiring setup on one of my bikes at this link:
https://www.bikeforums.net/electroni...b-charger.html