Here's a video shown on Japanese television that simulates a meteor of over 200 miles in diameter impacting the earth at about 44,000 miles per hour:
It destroys all life on the planet, covering the earth in a 'rock steam' that raises the temperature of the surface to 4000 degrees celcius. The oceans boil off completely in a month, leaving the surface bare. The impact crater is thousands of miles wide.
By comparison, the Chixilub (dinosaur-killer) object was about six miles wide. The P-T object was probably about 30 miles across. The latter killed more than 90% of everything on the planet.
On a somewhat related note, I find it fascinating that the cataclysmic destruction of Japan is a subject that occurrs at an unusually high frequency in Japanese programs, especially animated material. My feeling has always been that it's a subcurrent of post-Hiroshima feelings that drive this, but perhaps it's something deeper from a cultural sense.