Thanks Yurii. Just want to make sure I understand your answer. What I think you are saying is that the cross section area is more important than the surface area of the rod. Since the finned rod has MORE surface area, than a rod of equal cross sectional area, or of equal diameter, I was hoping that the rod would have an effective diameter larger than its actual diameter, instead of acting like a rod of LESS diameter. Please confirm that the rod will act more like a rod of slightly smaller diameter.
If you are considered about ohmic surface resistance (ohmic heating) - then effective diaemter is larger than actual diameter. Exact answer is complicated, since surface current distribution is not even across this surface.
If you are considered about impedance/reactance wire HF-properties (inductance, capacitance) - the only thing matters is fields distribution (eletric and magnetic) along and around this wire.
Since currents are flowing in inner parts of fins as well as outer parts, fields will be closer to wire (more compact), compare to scenario where inner parts do not exist at all.
Measuring effective diameter is quite easy both instrumental and using CAD-simulation.
Wire diameter affects characteristic impedance of 2-wire symmetrical open line. There is simple formula to calculate Z0 for line with given spacing and round wires diameters.
You take 2 finned rods and measure impedance between them: either directly or adjusting spacing between rods to get impedance of reference 2-wire round line.
It's very easy to do this using CAD (ANSYS HFSS or CST Microwave)