[Leviathan is... difficult to describe. Tifa's eyes round out, her lips parting to say something, but she holds the thought to think on it a little more.]

Like a giant blue water snake... dragon? With wings.

[Nailed.... it...?]

I'd say if you were to stretch Leviathan out, he'd be at least a hundred feet long, if not bigger. Shoots water tornadoes at you, hard to hit... Sound familiar?