The largest crab in the ocean is the Gann's giant mitten crab.

Macrocheira kaempferi, also known as the Japanese spider crab or goblin crab, is named for its resemblance to a tall-legged spider. Macrocheira kaempferi is the only remaining species of this genus, which is mainly distributed in the waters around Japan, and has been sporadically reported in the East China Sea.

This crab is indeed very large. But that's mostly the length of its legs; its torso is comparatively smaller, only a little bigger than a basketball! The average extended length of the adult killer crab's chelicerae is 2.5~3 meters, with the last four pairs of legs 2.1~2.4 meters! The shell is 25~33 centimeters wide. The limbs of the largest individuals were extended, with the chelicerae spread horizontally about 3.7 meters long, the pairs of legs spread 2.5 meters, and the shell was 38 centimeters wide!

The entire crab can weigh a total of 19 kilograms. However, most mitten crabs don't grow to this size, with fishermen harvesting mitten crabs with a spread of about 1.0 to 1.2 meters. While it's true that mitten crabs are meat-eaters, the spectacle of them feeding on small fish and shellfish and killing people off the coast to share is over-the-top.

And they mainly live in deep water between 50 and 600 meters, so even if any of them wandered to the shore, they would be scattered and would not be able to make such a powerful scene. I've searched the foreign web and can't find any recent news of anything like this, so it's likely that at least some of these media reports are suspect.