Robbin said a minimum of 6 AWG. THey are both true. I said nothing to the effect that its more complicated than ...robbins post.. as you are trying to make it sound by comparing the two in the same sentance.
said by tomupnorth: In any case, my ? was e.g. for a service panel (with rod) to a house (no rod) to a separate garage (no rod).
This wasn't your question and is not what I responded to. Maybe it is what you wanted to ask.
said by tomupnorth:If a ground wire exists back to the main service panel, can add'tl rods be installed at the remote buildings.
For separate buildings, where allowed, there must be a ground wire back to the next upstream panel. Yes, you can have a separate ground rod at the separate building, and in most cases its a code requirement.