Sizing depends on many things, so there is no one answer. If you're looking at transformer windings you come up with different sizes then you do when running a PCB trace.
Problem is a thermal issue: the wire will heat up. What class of insulation, how hot you can let the wire get is application dependent.
Also, if the application is heavily regulated, then there are often some additional requirement. example, Nat'l Elec Code has current capacity requirements for various types/sizes of wire
M Walter