Most of the stars in our universe are in binary systems. Hence, the size of the stars can be found when one star eclipses the other. When so happens, the change in the luminosity is measured and size is calculated. Another way is by measuring the luminosity of the star and comparing it with the Sun's luminosity. As the luminosity is dependent on the size of the star, the radius of the star can be calculated when compared to the Sun. Following formula can be used:
Star's radius/Sun's radius = (Sun's temperature/star's temperature)2 Sqrt[star's luminosity/Sun's luminosity].