Male, female, adult, juvenile?
From a camel to an elephant or a blue whale, I would think.
Even if the overall dimensions are larger, the plesiosaur has kind of the same body configuration as a camel, as opposed to an elephant.
Dig a big hole in the ground. Cover the bottom with hot charcoal. Wrap the animal in banana leaves and place on the coals. Cover with more wet leaves. Add more coals on top. Cover with earth. Drink lots, and come back when you wake up tomorrow.
Oh, I forgot ... throw in some garlic!