Not one big enough to cause another great extinction, but small enough to just turn a whole small country and parts of neighboring ones into a huge crater. Will the people who was evacuated out after NASA stated a warning try to rebuild the country after everything has settled down or do they’d become citizens of another country?
Edit: after reading the comments, maybe turning a small country into a crater is too much, what about just level the place, or in any way that make it uninhabitable for a period of time?
It’s an interesting question and would require a sophisticated model. As others have pointed out, the size, velocity, and density of the object and the amount of warning time are all critical variables. The target location is also a critical variable. Some countries have friendly neighbors who would want to help. Other countries have hostile neighbors. Some countries have a culture and infrastructure that would have a chance of executing a swift and semi-orderly evacuation. Many would not. Many populations would panic and there would be mass chaos.
This would be a fascinating thing to train an AI on to see what sorts of answers/hallucinations it comes up with. Apply for a grant and get on it!