>>> I wonder which one is the largest FF city..

FF3 Salonia, Huge city with forests inside its walls..
FF7 Midgar, You need to take a train to go through the different sectors..
FF8 Esthar, Probably the largest (you get lost easily)..
FF9 Lindblum, Looks super huge in the FMVs.
FF10 Zanarkand, Looks super huge in the FMVs.
FF12 Archades (the new one), Looks super huge in the FMVs.