What is The Difference between DC Cable Sizing and AC Cable Sizing?

In case of DC transmission and AC transmission the cable size for DC will be more due to its uniform voltage and current. But in case of AC. using traformers the current and voltage can  be changed. Thus reducing power losses. (stepping current down while stepping up voltage)

It does not matter much for AC or DC. The rms value of AC is equal to DC. However if AC voltage rating is high then you should cosider additional heating of cable due to AC loses.

for selecting a cable for ac,its rms rating is to beconsidered while for selecting cable for dc,its average ordc value is to be considered.

For DC, current distribution is uniform in all over cross section of cable so electrical cross-section = physical cross-section.

For AC, current distribution is non uniform due to the time varying nature of magnetic flux produces. So that Electrical Cross-section < physical cross-section. So Rdc<Rac.