What size wire......?

What size wire was used to go from the starter relay over to the alternator on a 73 Dart? Should the same size be used, when being replaced, or should I upgrade? Thanks in advance.

Original was no12, and that should be plenty.

More important, use a piece of high quality, flexible wire, because the engine MOVES. Route the wire with so it's got sort of a loop to allow for some flex.

I've seen more than one original break off right "in" the wire end terminal

Be sure the wire is a stranded type, many little wires. Also solder the terminals and heat shrink each end.

Just my $0.02..