You will need two 8 foot ground rods. One placed under the meter base and another one placed 6 feet to the side next to the foundation. If this is a home I highly recommend you install a 200 amp service instead of a 100 amp service. Not much more expensive and you will have plenty of room for growth.
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#4 bare copper connected to two 8' copper ground rods 6" apart driven into the ground.
It must be grounded outside with two 8' copper ground rids driven fully into the ground. One under the meter and one 6' to either side connected together with the proper size copper ground wire. The above answer leaves out the requirement for a grounding connection to a metal water pipe. The NEC actually makes the ground rods supplemental to the water pipe. We don't usually consider the water pipe as the primary grounding electrode but that is how it is treated in the code. Depending on the size of your service, the grounding electrode conductor to the water pipe may actually be a different size than what connects to the ground rods, which are allowed to be connected with 6 AWG copper regardless of the size of service.
You need 2 ground rods of 8' in length. One under the meter base and another one 6' to either side of the meter base. Drive them into the ground with just enough sticking up to connect the ground wire.
250 mcm
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.