I believe that, for a sub-panel, you're stuck with #8 to the ground rod. Remember, the ground rod is there for lightning, and has little to do with the service size.
The introduction to the code make it clear that the code is not a design manual, nor is it intended to allow for future growth, etc. With that in mind, I submit that, as a professional who is paid for his judgement as much as anything else, you should make these allowances, and exceed code minimums.
That is, run a big pipe -1" at least- and mount at least an A/C disconnect....at some future date, either the wire will go bad, or the customer will want more. How many garages are really workshops, after all? The $$$ for this improvement over code are minimal, and reap huge dividends down the line.
Or do you like digging? :-)