What is the factor that most limits the distance over which transmission of electrical power is practical:

Maximum practical voltage?

Maximum practical conductor size?

Acceptable frequency?

Other?

Cannot see that increasing the frequency would be of much benefit. An increase in frequency may make the transformers more efficient, but that will be offset by increases in loses through inductance and other transmission losses.

Conductor diameter and voltage are interrelated, although the greater significance regarding voltage is the level of insulation (whether this be simply an air gap or actual insulation around the cable).

Thicker conductors would allow higher currents (but aside from cost, one also has to consider weight implications), while higher voltage allows lower current to carry the same power.