Like Mr Gazza, I'm no expert in this field and agree that any information from the manufacturers themselves may not be wholly subjective as they have a vested interest and want you to buy their products.
For what it's worth, this diagram and explanation is from the Denso website.
https://www.denso.com/global/en/prod...condition.html
"
The smaller the electrode the more concentrated the electric potential at the tip of the electrode and the stronger the electric field that affects required voltage and the lower the required voltage."
Although this information comes from a plug manufacture, I do believe that electrode size and shape does affect spark generation and Denso's diagram appears to be backed up by independent research.
https://docs.lib.purdue.edu/cgi/view..._access_theses
This is not specifically about spark plugs but it does show how different electrode shapes affect the voltage needed to create a spark across any given gap and I don't see why the findings shouldn't also be relevant to spark plug design.