This is actually a similar question. How/what do i size gec for the x0 to the ground rod? Xfrm is 277/480 primary:120/208 400amp secondary.
similar, yet not. johnnymac was speaking of an equipment bonding jumper which in his case, as a service, is determined by 250.28. You are speaking of an SDS where 250.30 does reference us back to 250.28, but when you mention ground rod, we are now talking about grounding electrode conductors and not system bonding jumpers.
The grounding electrode conductor would be sized per 250.66, but if you have to fall back to a 250.52 electrode, I would think the #6 referenced in 250.66(A) would apply.
(woops a lot of folks answered while I was still looking up Code reference.. us old men can't remember so well)