If I do, then what gauge of wire should I use for grounding my main panel?

Is there any difference if I connect the ground to sewage pine or water pine?

Thanks.

Details. We need details.

First, where are you? US? (State?) CA? UK?

What specifically is an "outdoor power service box"? And why did you "ground" it? And to what?

In the US:
Sewage pipes are NOT suitable for use as an electrode, nor are they required to be bonded.
A metallic water pipe that is in contact with the earth for 10' or more must be used as an electrode and must be supplemented by another electrode.

There are quite a few important codes involved with doing a service. Did you get to know them before you started?

I live in Illinois. The power meter box outside of my property is grounded with 1/2in thick and 5ft long copper rod into the ground.

The question is: Should I still ground my main panel?

Thanks.

I'm not sure about Ill, but most places in the country now prohibit a grounding electrode connection inside the meter pan.

Also, a 1/2" x 5' rod is NOT code complaint and WILL get red tagged. You need and 8' x 5/8" ground rod.

YES, you need to run the grounding electrode conductors and any bonding conductors to the main panel/disconnect.

Where did you get your information from before starting this project?