One would need to know the size of the Service Entrance conductors to size the Grounding Electrode Conductor. When this is known use T. 250.66. You should not have to size this GEC larger than 3/0 cu or 250 KCMIL Alm. You may qualify for a smaller size based on the type of grounding electrode you find available.
Ground rods(s) Max would #6 cu, Concrete encased electrode Max #4 cu.
