The "size" or "event horizon" of a black hole usually refers to its Schwarzschild radius given by the formula R
s = 2GM/c
2, where G is Newton's constant, M is the mass of the object and c is the speed of light. A photon emitted at a distance greater than R
s will eventually make it out to large distances, although perhaps red-shifted a bit. However a photon emitted at or less than R
s won't make it out. So, looking at the formula for R
s there doesn't seem to be any reason why there should be a lower or upper limit to size of a black hole. Most galaxies will contain one or more black holes. Our own Milky Way has its own in the constellation Sagittarius.
Curiously enough, you don't need general relativity to derive the formula for R
s.
It was first derived in 1783 by John Mitchell, an English parson, using classical Newtonian ideas. Here is a web site with more information.
http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmiccs_michell.htmlOther sources about black holes can be found at:
http://en.wikipedia.org/wiki/Black_hole http://en.wikipedia.org/wiki/Schwarzschild_radiusLee H
(published on 08/06/2009)
