<If it struck the Earth a 300-metre object would not be a global killer: to wipe all life off the face of our planet an object would have to be about 1 km in size. But 300 metres is more than enough to cause widespread devastation.

It is estimated that the asteroid that killed off the dinosaurs, but definitely did NOT wipe "all life" off the surface of the planet, was about 10 miles (about 16 km) in diameter. Even a 300-meter asteroid might not "wipe out an entire country," unless it's a really dinky country like England. ;) I'm not sure what it would take to sterilize the Earth down to the last bacterium, but it would probably have to be the size of a small moon.

This is not to imply, of course, that such an impact would be something to take lightly.

A more realistic asteroid damage assessment can be found here.