Torn World: "Jellyriggers"

I should come up with sea monsters. Orion is such a huge planet that only the small seas between the smaller continents are navigable. The larger oceans are large enough that Earth could drown in one. They have super-hurricanes that make life on the coasts they hit impossible for sentient life to live on. (Unless they have mages to detect oncoming hurricanes and push them away.) So with oceans that big, the only limit for size of an animal is a function of how massive can they be before they collapse in on themselves?

The planet is so big that magic is the only thing keeping it from collapsing. Orion has the surface area of Jupiter, is hollow, and the inside is supported by adamantium. I made the planet so big because I wanted there to be hundreds or thousands of sentient species sharing it (humans, dragons of various species, other mythical creatures, and some unique ones). With a planet as big as Orion is, there's plenty of room to share.