All the stars in this photo are at about the same distance from Earth (some 25,000 light-years away). Which stars in this picture are the largest in size (radius)?

You did not add the photo, but the bright red stars are the largest in size (radius).

Explanation:

Red stars are giant, luminous stars. They are the stars with the largest size (radius) among all the other stars and usually have very low temperatures (below 500k). The brighter these stars appear to human eyes, the greater their actual size.

Red stars have rays hundreds of times larger than the sun, literally being the biggest stars in space. Despite their gigantic size, these stars have low density.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.