Grounding Rod Requirements
The NEC and your state codes do not reference size of service.
Washington Administrative Code 296-46B-250-053 (A)(2) Resistance of rod, pipe, and plate electrodes.
(3) For rod, pipe, and plate electrodes other than those installed in accordance with the exception in subsection (2) of this section, if a ground resistance test is not performed to ensure a resistance to ground of 25 ohms or less, two or more electrodes as specified in NEC 250.52 must be installed a minimum of 6 feet apart. A temporary construction service is not required to have more than one made electrode.
NEC 250.52 says &quot;at least 15.87 mm (5/8 in.) in diameter, unless listed.&quot;