We are adding a new ground that is 400 feet away, and we need to size it for 10 mega-Hertz. Can you please tell me the size cable that we need to install?

Is this you asking the same question at Yahoo?

I need to know how large of a cable I need to run for a 10 mega-Hertz ground 400 feet long? - Yahoo! Answers

My answer will be the same, grounding conductors are not sized by megahertz, but by the electrical system rating, without getting into too much detail.

We need more detail from you, what is this for, what is the electrical system details, etc.