1. (mythology) The Greek name for the Egyptian mythological figure. The Egyptian name is w:Apep, Apep.
2. (astronomy

   ) A near-earth asteroid designated with the permanent number 99942. There is a Wikipedia article on W:99942 Apophis, 99942 Apophis.

525 feet

The answer to this can be found in the links I will place below.

About the size of three football fields.

Neither; Apophis is an asteroid, and not especially large. It would bear no mention at all, except for the fact that on Friday, April 13, 2029 it will come within about 20,000 miles of the Earth, closer than the orbit of geosynchronous satellites. But it will miss.