The NEC only requires 5.261 (mm)2 for the protective down conductor
and 13.30 (mm)2 for the bonding conductor between electrodes. Since
those sizes are at best a bad joke I was hoping to elicit best
practice advise on what size the conductors should actually be as well
as advise on how to accomplish the bonding of the interior single
point grounding buss bar to the exterior grounding conductors and
Grounding Electrode System.
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In what way is #6 a "bad joke?"

Do you expect it to vaporize and set your roof on fire?