Which one? The stock Red w/blue one at the original starter relay on the fender? Or a new one from the relay down to the new solenoid?
The original one can be left alone as it does not see any more current than it ever has.
The new one from the relay to the solenoid would likely benefit from being 14ga, although it's actual current is workable with 18ga, you're better off with a little overkill for the long haul.
And if you route yours around hither-and-yon for a clean look, the longer the wire the larger gauge it should be anyway.
There's probably a specification out there for what a solenoid pulls, but it's probably less than 10a for our type of starter. But amperage may go up as heat increases, and if the starter is ever called upon to do a LOT of cranking, that little bit of overkill would have been an even better idea.
But that's all just a pure guess with zero knowledge backing it up.
I say for long life in a fairly hot environment, some common 14ga with maybe even a heat-resistant sleeve covering it would be your hot ticket.
Paul