This is what would happen if a comet smacked into Earth
Size of the comet or asteroid not the only thing to consider as the brunt of the damage would come from gases released into the Earth’s atmosphere

With the Delta Aquarid meteor shower going on right now, and the crowd-favorite Perseid meteor shower hot on its heels, the next few weeks are going to be the prime time to watch some shooting stars light up the night sky.
But while dust-sized bits of comets sizzling through our atmosphere put on a pretty awesome show, the consequences of Earth confronting one of these comets head on could actually be pretty disastrous.
It’s not something that you should be worried about, of course. NASA is on the lookout for any cosmic objects on a crash course with our planet, and they’ve found that the chances of us colliding with a comet or asteroid anytime soon are virtually zero.
It's still pretty interesting to think about though. What would happen if one of these ancient, celestial chunks of ice, dust, and rock smacked into our planet?
The 16-mile wide Swift-Tuttle comet — the progenitor of the Perseid meteor shower —hurtles through space at about 36 miles per second, more than 150 times the speed of sound.