How do scientists measure the size of other planets in the solar system?
Planets reflect light from the Sun. The planet is seen as a disc. The angular spacing of the disc and the planet's distance determine the size (diameter) of the planet.
It is assumed that the radius of the Earth is small compared to the distance of the planet.
When the planet is over head, in the night sky, the angular spacing of the planet disc is
Sample data> Mean distance of Mars =22790993 km and
Mar;s diameter = 2(22790993)tan(0.017/2) = 6762 km, nearly.