What gauge wire is used for dynamo hubs and lights?
Won't matter much, 22AWG is rated at about 1AMP for power transmission and 7AMP maximum current (i.e. that is around the current insulation starts to melt), is plenty large enough to carry the current from a dynamo (a few hundred milliamp's) and provides a strong enough wire that it won't break easily when in use.  A size or two smaller or larger won't hurt.