Measure or reference the luminosity of the star and its surface temperature. As an example, Betelgeuse, which forms the left-shoulder of the constellation Orion, has a luminosity of 38,000 L-sun and a surface temperature of 3,400 Kelvin (K).
Convert the luminosity from L-sun to watts by multiplying by 3.8 x 10^26. This figure is scientific notation to avoid using an excessive and confusing series of zeros. The notation "^26" means "to the power of 26." In the example, Betelgeuse would have a luminosity of 1.4 x 10^31 watts, i.e., 38,000 * 3.8 x 10^26.
Write out the stellar radii formula:
Stellar Radii = squareroot [Luminosity / (4 * pi * Stefan-Boltzmann Constant * (Temperature^4))]
"Pi" is a constant, measured at 3.14. The "Stefan-Boltzmann Constant" is measured at 5.7 x 10^-8.
Plug your values into the formula to solve for stellar radii:
Stellar Radii = square root [(1.4 x 10^31) / (4 * 3.14 * (5.7 x 10^-8) * (3,400^4))]
Stellar Radii = 3.8 x 10^11 meters
This equates to about 360 billion meters across.