Yeah, Onigashima varies wildly from panel to panel. If we look at this structure here:
It looks hundreds of metres tall scaling from the 56m tall sunny.
But in the next page, it is barely 50m tall
Oda is hardly consistent with this stuff, so I would take the multiple km scaling with a grain of salt.
Safe bet is Marineford being comparable to Onigashima, especially if it fits snugly in the capital.