Is Ishgard the largest city state?

Obviously we don't know for certain, but from the sound of things it looks like most if not all of the new Heavensward zones will be located around Ishgard. They said that the new areas will be approximately 1.5x the size of what we have now in terms of map size. Add that to all the castles around Coerthas already in-game and that would make Ishgard/Coerthas the largest city-state in all of Eorzea in terms of land and population (about 3x as big as Black Shroud, La Noscea, and Thalanan).

Do you think this is true? Or will we probably be seeing other, new lands as well on our journey from 50-60?

We will be traveling beyond Ishgard's borders. For example we will be heading into Dravania at least.

Judging by artwork shown, Ala Mhigo actually looks to be the largest in terms of physical size - it's an enormous castle city that dwarfs Ishgard's citadel on a crag, and the lore of the game has stated from the very beginning that Ala Mhigo was once the most powerful and technologically advanced citystate in Eorzea. Of course concept art is never 100% accurate to how thigns actually turn out in the game, but even with the Garlean occupation, I'm still liable to regard Ala Mhigo as the largest citystate in Eorzea.

If you are including the Dravanian Outlands, I'm sure Ishgard and the dragons have very different opinions about what belongs to who.