Passage: At an estimated 1.15 M☉, IK Pegasi B is considered to be a high-mass white dwarf. Although its radius has not been observed directly, it can be estimated from known theoretical relationships between the mass and radius of white dwarfs, giving a value of about 0.60% of the Sun's radius. (A different source gives a value of 0.72%, so there remains some uncertainty in this result.) Thus this star packs a mass greater than the Sun into a volume roughly the size of the Earth, giving an indication of this object's extreme density.The massive, compact nature of a white dwarf produces a strong surface gravity. Astronomers denote this value by the decimal logarithm of the gravitational force in cgs units, or log g. For IK Pegasi B, log g is 8.95. By comparison, log g for the Earth is 2.99. Thus the surface gravity on IK Pegasi is over 900,000 times the gravitational force on the Earth.The effective surface temperature of IK Pegasi B is estimated to be about 35,500 ± 1,500 K, making it a strong source of ultraviolet radiation. Under normal conditions this white dwarf would continue to cool for more than a billion years, while its radius would remain essentially unchanged.

Question: How can the radius of a white dwarf be measured?
from known theoretical relationships between the mass and radius of white dwarfs