The size of the planets What calculations were done to discover the size of all the planets?
 A: Planets in our own solar system are measured directly.
You can measure their angular diameters with a telescope and their distance by measuring the maximum angle they make from the sun.
More recently the accurate distance to the inner planets have been measured by radar
A: For exoplanets, the technique used depends on the aspect of "size" being measured.
Masses of exoplanets are estimated by using doppler spectroscopy to determine the acceleration of the planet's parent star caused by the orbiting planet, while diameters are estimated by observing the percentage of a the parent star's light that is obscured by the planet as it crosses between the star and the observer.
Masses are actually minimum masses and diameter can only be determined for planets which "transit" their parents. Both estimates require knowledge of the star's properties—for mass, stellar mass, and for diameter, stellar diameter—both of which are estimated from observation of stellar properties (spectra, known distance, etc.) and stellar evolution. 
