The standard alternator to battery cable is 6mm^2, but if I was going to replace the equivalent on my saff, I'd use a bit bigger, say 8.5mm^2 (certainly if using a 90A alternator).

25mm^2 cable is suitable for the engine to battery earth.

The standard battery tray to battery earth is about 6mm^2, but if I was going to replace the equivalent on my saff, I'd use a bit bigger, say 8.5mm^2.