An approximate value for the surface area of the earth is to use 4πr2 (surface area of a sphere) where r=radius=3960 miles approximately. So the surface area is about 197 million square miles (510 million square kilometres).
But the earth is actually an oblate spheroid (not quite a sphere) and is not flat, so all natural mountains, valleys and hills would need to be taken into account. The seas (including the frozen seas) cover about 71% of the earth's surface and may be regarded as flat compared to the land (except for ice mountains), but water evaporates, ice melts, and the land is changing its geography continually, so it would not be possible to give an exact answer.