This space rock is asteroid 2005 YU55, a veritable mini-world roughly 1,300 feet (400 meters) wide — nearly four football fields across — that will zoom by Earth inside the orbit of the moon.....snip~
From the article it stated 400 meters wide and nearly 4 football fields across. Moreover they did say that it was going to pass inbetween the earth and the moon. Guess that gave them the idea that it was the size of a small planet. So much for our calculations.