I while ago (ok, some years ago) I stumble across a NASA web page that let you calculate how big the impact of a meteorite is going to be. You simply fill in some numbers like how big the 'projectile' will be and so on and it will calculate a few things for you like how big the resulting crater will be plus a whole lot of other useful information like the big the earthquake will be etc.
The URL is http://www.lpl.arizona.edu/impacteffects/
Just as an example, I have specified an object 50 meters in diameter (average) and it will create a crater over 2km wide and 500 meter deep. You don't want to be close to the impact site when it hits. Even at 50 km from ground zero there will be a 5.6 earth quake (on the Richter Scale). The explosion will be equivalent to a 33.8 Mega ton blast. It will be about 3 times as bright as the sun.
It is both fun and very scary how this can happen at any moment and there is about f@$all we can do to stop it if it should happen.