Thank you for the problem, professor KS. I finally got to it tonight.

Based on a website calculator, the length of the arc from Tijuana to Brownsville is 1,263 miles. I added an extra 12 miles to reach to both coasts for a total of 1,275 miles. After little geometry and trig, and assuming that one can actually resolve land features at/near their horizon, I calculate that that height above the Earth is around 25 miles, give or take.

Of course, that's not outer space, but at that height one would need to turn their head through about 170 degrees to see the darn thing, and near the horizon it would be difficult or impossible to perceive. So, I'm guesstimating that to take in the entire thing with reasonable resolution it would be near or above the boundary of the atmosphere and where space is accepted to start.

I enjoyed this. Do I get an A?