The thickness of the Earth's atmosphere is estimated to be what?

I don't like the wording of the question. We have a good understanding of how the atmosphere changes with elevation. But the Karman line is arbitrary and chosen at 100 km because, well, a vehicle in sub orbital flight does not get much air resistance there and does not have the ability to use jets, must use rockets, with its own oxidiser. As such it is no physical boundary line.