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.)