The size of ground rod you need for 100 AMP electrical service is K. GROUND ROD AND CLAMP Rod 5/8" x 8
You also need a #8 copper ground wire to meet code for the ground on the 100 AMP electrical service.
If you use copper service entrance wire you'll need #6 copper wire which is also known as 3-3-3-5 copper SER cable.
If you want to use Aluminum Service entrance cable it needs to be #2 or #1 aluminum service entrance wire to meet code.