Reno:
The siding is NOT any part of the grounding electrode system, the discussion is regarding bonding the siding, or that's what I thought.
Bonding conductors are sized based on the OCP of the circuit that is likely to energize the conductive material. Hence, as I said on page 1, the largest OCP IMHO would be the service main, and in actuality the OCP at the utility primary. With the al. siding 'floating'...it could become energized by a fault, ranging from a 15 amp branch, to the 100+ service main, or the utility primary. Now, bond the al. siding, a fault trips the branch feeder, or...the al. siding vaporizes.
Sticking with a resi job, IMHO other that the service, the largest OCP would likely be the HVAC, or possibly a hot tub at 50 amps, hence a #10, or good old #6 for a 200 amp service.
Again, this scenario is not (to the best of my knowledge) addressed within the NEC directly, but..."likely to become energized" could be interperted many ways.