My opinion is that conductors impedance is a negligeable part of total impedance. Main grounding resistance or impedance is due to current transition from conductor (rod, plate, grid, pipe, etc) to ground, plus current spread conduction into the ground itself.
   Transition resistance depends mainly on electrode size and surface quality and on ground type, conditioning and humidity degree. Conduction depends on ground conductivity and electrode equivalent radius as well.

Julian

Julian's right.  The earth and the conductors / rods / whatever are all superb conductors, we just can't hook them together in a low-impedance way very easily.

Be careful with choosing -- remember the code requires that underground metallic water piping and structural steel must be bonded to a grounding electrode too in almost all cases, in order to prevent potentially lethal potential differences during fault currents or lightning activity.

A cheap and effective grounding electrode connection in my experience is to trench 18" to 24" deep, 4" wide, and 25 feet long, somewhere convenient.  Lay a bare #4 copper conductor on 2" thick spacers in the whole length of the trench, and fill 4" to 6" deep with concrete.  Connect to the conductor at the end with an exothermic weld.  I have never failed to get less than one ohm with this method, which I learned from a Westinghouse systems engineer friend, even when three 8-foot driven rods couldn't get me below 5 ohms.  Best of all, I found that if I timed things right on the construction site I could use somebody else's ditch witch and concrete if they were already there, saving me the trouble of rounding these up for such a little ditch and pour.  Usually cost me a sandwich and coke.

Best of luck!

Old Dave

Dave's ground electrode is also known as a Ufer ground, if you want to search for more information on it.

Given that soil resistivity is generally the largest single factor in determining resistance, increasing the surface area of the grounding electrode is most effective.  Concrete is fairly low resistivity when moist, and its volume in a Ufer ground presents a very large surface area in interfacing to the soil.

Similarly, augering a small-diameter hole around the ground rod and backfilling with low-resistivity material such as bentonite clay or a proprietary mixture (ie. Erico GEM) is also effective.  Since the surface area is proportional to r^2, small increases in diameter greatly increase the surface area.

From this point of view, a pipe could be quite effective, as a 1-1/2" pipe would provide four times the surface area of a 3/4" ground rod.

Typically the diameter of earth rods has little effect on the overall earth resistance of an earthing system. The most significant factor is the soil resistivity, and also the area of the earth mat or grid. Earth resistance tends to change through layers of the soil. For example the top layer may be 100 ohm-metres (good soil), second layer may be 500 ohm metres (debris and rock)etc. For normal applications a two layer soil model is typical. Sometimes the lower soil level may have lower resistivity than the top. This is the reason for driving deeper rods to get to this lower soil resistance. Otherwise general rule stick to 1 metres rods and lay them out in a grid with as wide a surface area as possible. Should also be trenched. In the UK to 600mm below ground i.e. below the frost layer

Just remember your pipe will disintegrate a lot faster than a solid rod of the same diameter.