I haven't taken an original harness apart, so I'm pretty much guessing but I'd say it's either #14 or #16 for the most part. Keep in mind that there is different size wires depending on what/where they go and what fuse they come from. The other thing to keep in mind is on the engine side, most of the harness is wrapped and not exposed to the elements. It separates at the firewall plugs and whatever device it's attached to. Even behind the firewall, a lot of it is wrapped. Also, on original cars (post 2201 standard leaf spring cars), some of the harness is Ford Autolite and some of the harness is Lucas. There are short connectors with a Ford plug on one end and a Lucas plug on the other separated by a couple inches of wire to connect the two together.
So it kind of depends on how closely you want to replicate it as to what you want to use.
Here's a pic of the firewall, less engine with the harness in place:
Larry