Approximate Size: 320 metres
The mountain-sized side-quest which initially terrified Final Fantasy fans with the rumours of a 72-hour boss fight is a slightly controversial entry for two reasons.
Firstly, the actual size of the mega-tortoise isn't officially listed anywhere (that I could find, at least), making the 320m length something of a guess based on how it compares to both Noctis, and the other entries in this list.
Secondly, its hard to tell which is bigger: Adamantoise, or Leviathan. Both beasts are labelled as a category 5 size in the official FF XV wiki, though we get no more detail. It might well be that Admantoise is bigger in terms of weight, while Leviathan is longer.
Ultimately, Adamantoise beat Leviathan to the list not only for its superior HP and strength, but because it is, quite literally, a mountain. Both boss fights are immense and make our hero feel insignificant in scale in equal measure, but while the Leviathan battle offers some of the most epic music in FF XV, there are few reveals more imposing than when Adamantoise first stretches out his neck from beneath the enormous rock on its back.