The hole is very big, but it's not as big as Onigashima.
The mountains surrounding the hole look identical to the mountains surrounding Onigashima. There's nothing suggesting that they are any larger than those mountains (nor that they are any smaller), so the safe assumption is that they are comparable in size.
Here's Law's hole:
Here's Onigashima:
It's a big hole, but I really do not think it's as big as Onigashima (if anything, I think it's significantly or considerably smaller).
I don't want to downplay Law's feat either, but unless we get a direct comparison between Onigashima and the hole that suggests they are comparable in size, I'm not going to assume that they are.
At the very least, the hole is the size of several mountains.