I just had a look on the map and found Porto Le Gratia and Porto Vena are almost the smae size. I think Porto Cavo is the biggest because it also includes the third longest runway in the game. I also essentially splits Capite Est and West as it goes from one end of the coast to the other and for about half of the base the border between east and west runs along side it.
I have taken a look at the areas of the bases using a 60” TV. I found the three biggest bases in actual area covered are Corda Dracon in 3rd, and a tie for 1st between Porto Cavo and Tridente. Vena and Le Gratia are literally the same size in terms of base territory according to the map. In terms of bases in proportion to the size of the province, Cima Leon is in 3rd, Falco Maxime in 2nd, and Corda Dracon in 1st. There are very few areas in the province where you are not on base territory. In terms of which bases can be seen from the most places, I have looked and deemed Puncta Sud third, Vigilator Sud 2nd, and Falco Maxime 1st. Puncta Sud can be seen from most places south of Vista Dracon, Vigilator Sud can be seen all through Fonte and Dracon, and as far north as Cava Grande Secunde, and Falco Maxime can be seen from almost anywhere in Medici. If I was counting DLC, The eDEN Airship would crack all 3 lists too.
On a side note, the wiki states Vulture and Griphon to be one of the largest bases. Griphon is about the same size as Vulture, which in turn is about 5/8 the size of Tridente. The smallest bases not including oil rigs and outposts from a basic standpoint are Cima Leon Centcom, Cima Leon Silo, Cava Grande Secunde, the Vigilators, and Le Tutor.
The thing about Tridente is that you have to count only the area of the piers and the shore area. That makes it a bit smaller. And some bases have more area than they look like they have, because they're on multiple levels (overlapping runways and caves). And the mines are just industrial deserts with proportionally relatively few military stuff to cover them.
I don't know. It's hard to tell in terms of the amount of usable ground area, military equipment, area and usable floor space of the buildings, amount of vehicles, the potential size of inaccessible underground areas (if any) and so on.