ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades Earth Rod Size??

What actually decides the size of a driven earth rod?.
Sure I know it's written in the Regs in most countries, as to the size, but do you stop at that?.
Surely soil types vary from area to area, take for instance between Rotorua and here in Ashburton.
(Rotorua has a very sulphuric soil), here the soil is very stony.
How do you get good contact?.