Any tips of build giant characters/npcs?

Recently I’ve been playing subnautica a lot and I’ve seen a youtube short about a giant 5km long Leviathan from a mod for subnautica. So naturally I started to brainstorm about this. How could a giant NPC like that work? Would it have one capsule component/collision cylinder or many linked around it’s body, is that even possible? How would it work if it was out of water? Like let’s say something like a Dune Arrakis worm or something? Any tips/ideas?