The egg of a large modern bird is a very large cell, not the largest possible biological cell since there are very long nerves in squid, but it's likely the most massive modern cell.
But, what about the cells of prehistory? Many dinosaurs were far larger than modern birds, but I had trouble finding information on just how large their eggs could be. I was also surprised that the eggs were not bigger than those of modern birds. It seems the ratio of egg size to dinosaur size was much smaller... or has that even been studied?
Maybe the large eggs were all destroyed?