D&D 5E Anyone worked with Colossal/Mythical monsters before?

[dave2008]

I haven't thought much on actual sizes, since these are so monstrous. For example, imagine a shark so massive it leaves a Tsunami when it rises to the surface, or Charybdis large enough to make a whirlpool so big you cannot see the opposite side.

Something that large is really a different problem all together then and a typical combat encounter set up is not going to work. Oofta and Mechatarrasque have the right approach for such an encounter. It is more like a 4e skill challenge.

To defeat such a beast you need a special artifact (I win button), or the PCs have to fight through parasites / minions to get to some weak spot. You can't "fight" such a beast with the tools D&D traditional provides.