Copper? In plain earth? That will only give you complete corrosion in, depending on conditions, not even a whole year.
Usual standard is zinc-coated steel, 10mm (or larger, depending on stuff like attached lightning protectors, the diameter of the supply line and a number of other factors).
The connector wire is copper, sized from #8 to #4. The ground rod is 5/8" galvanized steel. That's the standard ground in the USA since water pipes went to PVC. And the copper will last underground indefinitely.
As mschuster91 said, there is always a risk of hitting existing lines when driving a rod in the ground.
Usual standard is zinc-coated steel, 10mm (or larger, depending on stuff like attached lightning protectors, the diameter of the supply line and a number of other factors).