What is the minimum size of a planet that could harbor human life? You get to decide what the atmosphere is, but I'm reasonably sure that O2, CO2, and H2O are necessary components of the atmosphere. Assume the planet's surface is flat. I'm not sure what the soil and rocks the planet are made of, even though I know that plays a role in the planet's ability to harbor an atmosphere. (And the atmosphere must be hospitable to plants as well, or else everyone is going to die.)