Look at the diagram the skull could easily cover the entire hole but apparently its halfthe size of onigashima
Then it is the 1/3 of Onigashima, which would be still big.
We see the mountains around Wano, they are pretty huge, yet they can´t fill up the hole:
And to made it clear, the diameter of the hole is just secondary, more important is how deep the hole is and it clear far far far larger then Onigashima:
And we don´t see even the end of the hole, based on Attack potency this feat is pretty crazy, it also had a previous calc who put it into large island lvl, a feat was already made and posted by Steven:
Assuming it's not 4km, but 16km (to fit Onigashima), and 5.7km deep (base of Onigashima)
Pulverization: 61.13 Gigatons (Island level)
Vaporization: 7.3 Teratons (Country level)
It is not big as Onigashima, but it is easily 30km deep which make the feat even more impressiv, I think we should get new calc this weekened. But for sure this feat is island-large island lvl, pretty sure the new calc will made it even more clear.