This question investigates the most extreme differences in the sizes of stars. Compute the ratio of the radii of a M supergiant star to that of a...

The temperature, radius, and luminosity of all stars (not just main sequence stars) are related by this equation:

Where L_0, R_0, and T_0 are based on the Sun, which has a radius of 6.96*10^8 meters and a temperature of 5780 K.

From there, we can solve for R:

Plug in the numbers for the supergiant:

Similarly for the dwarf:

Despite one being a "supergiant" and the other being a "dwarf", they are not that different in size, because these terms are actually about luminosity and temperature, not about size per se. (Also, they're both huge; the distance from the Sun to the Earth is about 1.5*10^11 meters, so both of these stars would engulf us if they were swapped out for the Sun.)