Big. No other way to describe these things. According to Wikipedia:

"The Japanese spider crab, Macrocheira kaempferi, is the largest known arthropod;
fully grown it can reach a leg span of almost 4 m (13 ft), a body size of up to 37 cm (15 inches)
and a weight of up to 20 kg (44 lb). The crab's natural habitat is on the bottom of the Pacific Ocean
(some 300 to 400 m deep) around Japan, where it feeds on dead animals and shellfish.
It is believed to have a life expectancy of up to 100 years."