This may help. Example from a '76 Alfetta. The wire on both terminals is 10mm, but the positive has an extra sheathing for protection for the long run to the front of the car. Note - the negative is attached to a chassis nut behind the rear seat, with a 13mm bolt. It has never been an issue.
Sorry about the resolution - phone camera.
1972 Fraud Escort Panel Van (Alfetta drivetrain), 1976 Alfetta Sedan, 1987 Alfa 90 Super