Bottom line...you can't go too big when it comes to battery cables. But 12v is ok with smaller wiring than 6v.
Then it becomes a matter of measuring the length of cable you need. And that is also determined by the amperage of your system and what you're powering, how you're charging it, etc.
Here's a chart:
Recommended Length and Amperage for Battery Cable while maintaining a 2% or less voltage drop at 12 volts:
Battery Cable Size 50 Amps 100 Amps 150 Amps 200 Amps 300 Amps
6 Gauge (AWG) 11.8 ft 5.9 ft 4.4 ft 2.9 ft 2.2 ft
4 Gauge (AWG) 18.8 ft 9.4 ft 6.3 ft 4.7 ft 3.1 ft
2 Gauge (AWG) 29.8 ft 14.9 ft 9.9 ft 7.4 ft 4.9 ft
1 Gauge (AWG) 37.7 ft 18.9 ft 12.6 ft 9.4 ft 6.3 ft
1/0 Gauge (AWG) 47.5 ft 23.8 ft 15.9 ft 11.9 ft 7.9 ft
2/0 Gauge (AWG) 60 ft 30 ft 20 ft 15 ft 10 ft
3/0 Gauge (AWG) 75.6 ft 37.8 ft 25.2 ft 18.9 ft 12.6 ft
4/0 Gauge (AWG) 95.2 ft 47.6 ft 31.7 ft 23.8 ft 15.8 ft.
Thing is, we know you have a 1950 Chevy. Truck, car, bus, cement mixer? We don't know where your battery is, whether you have a generator or an alternator or anything else, so we really can't do much more than to give you some guidelines at this point.
