Battery cable (+) question
 

I've done the battery relocation project to the rear using a M3 sedan battery cable. Problem is, the cable is not in the best shape, especially near the (+) terminal. My car keeps shutting off intermittently. Cause is a poor connection/frayed cable near the battery terminal, especially the thinner cable that goes to the fusible link. If this thinner wire is interrupted, your car shuts off. Lights, horn, etc still work, but when you crank, it does not start.
Anyway...

I do not want to buy a new BMW battery cable. Instead, I want to go to Lowe's or H.D. and get ___ amount of feet of cable and then replace the OEM cable with the generic cable.

Questions:
1) What gauge should I use for the main positive cable? I'm referring to the cable that runs through the firewall, passenger side; bellow the passenger seat, and to the battery on the right rear of the car.

2) What gauge is the fusible link cable AND the actual fusible link?
Thank in advance.

Go to your local auto parts store, they'll have battery cables and crimps. Make sure you wrap the repair really good and you should also use heatshrink.

Whatever you decide, I wouldn't skimp on any of the wiring between the battery and the alternator.

Should the battery disconnect from it at a high engine speed, the resulting "load dump" could prove very destructive to the car's electronics. It is the most severe fault a designer must guard against, and some fully don't, or don't, because of the cost.

My plan is to replace the entire battery cable. Again, from the jump start block in the engine bay all the way aft to the battery. I'll guestimate the length but I do not know what gauge wire I need.

The ETM shows 25 mm^2 RT for the main battery / fusible link wiring. It doesn't show a value for the link itself. The charts I find convert that to AWG 3.