While the comet will come closer to the Sun than the Earth, it will never actually cross any point in space that is occupied by the Earth. The comet will come up from the south, go over the top of the Sun and then plunge down again. In March and April 1997, it will be easily visible only from the Northern Hemisphere. There have been reports that this comet is very large. Actually, the heart of the comet, the nucleus, is obscured by the dust and gas that forms the head of the comet.
Nobody knows how large the nucleus is.
It hasn't actually been seen yet. Nuclei of comets range in size from a few miles or smaller to over 100 miles in diameter. The brightness of the comet is not always directly related to the size of the nucleus. This is typically because only a fraction of the surface of a comet's nucleus is active. It is possible that this comet has a small nucleus with most of its surface emitting dust and gas.