In Flight Simulator, the earth is defined as an “oblate spheroid,” an ellipse rotated about its minor axis. In terms of shape, it's much like a sphere that is a little bit fat around the equator. In Flight Simulator, the earth has the following dimensions:
Equatorial diameter=12756.27 km
Equatorial circumference=40075.0 km
Polar diameter=12734.62 km
Polar circumference=40007.0 km