This is for my 1985 300D.

I would like to keep a jumper wire in my car for use in the case that I might sometime get stranded with a failed ignition switch and/or neutral safety switch and hence, the starter not cranking or the glow plugs not glowing.

I would be using the jumper wire between the battery + terminal and the screw connections in the terminal block on the right front fender.

This might seem like a silly question but I'd rather ask and avoid getting burned or starting a fire:
What gauge of wire is adequate to carry the load when using such a jumper wire?

I'd appreciate any clarifications on this. Thanks-