Observe a binary eclipse of two stars. To witness an entire eclipse, you will need to track the initial position of the first star and watch closely until it passes to the opposite side of the second star.
Start your stopwatch when the first star is in its initial position. Continue timing until "star one" has fully eclipsed the second star. Record this time.
Measure the velocity of the star using the Doppler effect. The Doppler effect measures wavelengths emitted from an object. The velocity is dependent upon the external property of the motion of star one through space.
Find the diameter of the star using the equation Distance = velocity x time. The diameter can be used to find the circumference of the star by multiplying the diameter by pi. This will give you the size of the whole star.