When choosing spark plugs one has a lot of options. One of the differences is the diameter of the electrode tip - some spark plugs have a large flat tip, and newer models often have a really thin tip. There are many options in between as well, I've seen exact recommendations for certain models.
As far as I know the spark usually is emitted from the sharp edges of the tip, as the electrical field is the strongest at those points and discharge happens there. Sometimes grooves are added to the tip to increase the length of these edges. So what is the effect of different diameters, is a thinner tip always better?