Re: How big are stars?

The size (diameter or radius) of stars varies depending on their mass, 
their energy output (called the luminosity) and their surface temperature.

The largest stars are red (cool) supergiants. They are about 1000 times 
the size of the sun.  The sun is about 100 times the size of the earth or 
diameter of the sun=1.4 million km. The smallest regular stars are red 
(cool) dwarfs.  They are about 0.1 the radius of the sun.

Besides regular stars (i.e. stars that produce energy by nuclear 
reactions), there are also three types of  stellar remnants (i.e. objects 
that have used up all their nuclear fuels).   They are White Dwarfs, 
Neutron Stars and Black Holes.
White Dwarfs are about the size of the earth, Neutron Stars are about 10-
20 km in diameter and Black Holes have a radius in km = 3x mass of the 
star in solar masses. 
 
radius squared of a star = Luminosity/(4 pi Stefan Boltzman constant x 
Temp to the 4th power)
[R^2=L/(4 pi sigma T^4)

You can find the formula in most Introductory Astronomy books on the 
Chapter dealing with the properties of stars. 

For example, Foundations of Astronomy by M. Seeds (4th ed) p174