It’s a question an old high-school teacher of mine once posed to show just how much people over-estimate man’s place on Earth.
You are correct in your estimate. A hole 1 mile square and less than 1/2 mile deep would be a grave large enough to dump all six billion people.
It’s hard to imagine how microscopic a speck that is.
Most people are math-challenged and give a knee-jerk response like “a hole the size of Texas !”
I accept your estimate of a hole even smaller than I estimated on the grounds that I made my calculations on the basis that all humans were adults and Americans. This would delibrately give me a hole too big. Don’t want any feet, hands or assorted body parts sticking out above ground.