Approximately 6 square feet in the part of the country we live. The range across the US is 2-10 square feet depending on climate zone.
Those numbers assume an 8’ ceiling, so the cubic feet range is 16-80.
In other words, a human inside an R30 insulated box 6’x2.5’x2.5’ won’t require any supplemental heat to maintain 72*F inside in Winter. It also means that you can maintain the temperature in a 8’x10’ room if it has 12 people in it.