It's a little larger than the estimated size of the meteor that hit in Tunguska in 1908. That one flattened an estimated 80 million trees in an 830 sq. mile area. Some have estimated that the Tunguska explosion (it exploded before striking the earth) released the equivalent energy of 1,000 Hiroshima bombs. It would be a very bad day if a similarly sized rock hit a populated area.
Well, it is one thing for an asteroid that large to hit earth. It is another for an asteroid to start out that large before it breaks up in the atmosphere. I am not sure how big the remnants would be after it broke up and partially burned up coming down but it would no longer be anywhere close to building sized after falling through our atmosphere, would it?