Hummmm #6 is SOP
You usually plan wire for the following significant factors:
Max Amps
Min voltage drop
Temp rise (bundled or out in the open?)
Length
Weight
Durability
I would size the B-lead for the alternators full 60 amp rating not the intended/estimated 20 amp load, simply because the alternator can get to the full +60 amps. Did you include the initial battery charge current right after start? That can be 7 amps or more?
My recommendation, #6, assuming 60 amps cont over 6 feet. Why? First it's just plan more robust, bigger, tougher and the crimp will be easier to make with more contact area. The wire and terminal will be less likely to break from engine vibration and fatigue. It's like the ground. The run is short so super-size it and get min loss of voltage and take a small weight hit. To be a minimalist you could live with a wire as small as #8, especially since you are unlikely to get near 60 amps. Going smaller, say to #10 "would work" but the wire would get very HOT at 60 amps! (like 117F over ambient in a small bundle)
If you want to stick with the min 20 amps design, you could use #10 awg. You could use a #14 bare min, but that would be a poor choice for all the above criteria. It "would work" but get real hot and have almost 5% V drop. The min for 20 amps I'd use would be #10. Again don't forget about battery charging load. Actual load might surprise you? #10 would still work for 30 amps.
You might think a smaller B-lead wire size for 20 amps would act as a fuse and melt at 60 amps. Well not exactly but regardless you never want 6 feet of red hot wire. Of course, you should have a CB/Fuse on the buss/battery end of the B-lead to protect from shorts.
Building a racing record braking plane I might save the weight, but for a RV-4, go with durability. Also if you where saving weight you'd go with a 40 amp alternator or an accessory driven B&C 20 amp permanent magnet dynamo. 60 amp alternators weigh several pounds more than the 40 amp units. The good thing about having a 60 amp unit, you have room to grow; also a 60 amp alternator should run cooler, which is goodness.
