The Great Dark Spot is almost the same as the Great Red Spot because they're both anti-cyclonic storms. But unlike the Great Red Spot, which can fit 2 Earths, the Great Dark Spot can fit 1 Earth. So...
they were probably made by storms
That spot that you refer no no longer exist. It was always decrasing in size before it was gone meaning there was no absolute size.
Yes. Yes it is.
it is thougt to be a hole in the methane cloud