Dan Carlson created what is probably the most comprehensive pictorial of relative sizes of starships in various sci-fi books and movies: http://www.people.iup.edu/pnwm/comparison.gif - via reddit
Update 1/1/07: Jeff Russell has a comprehensive website on starship dimensions: Link - Thanks William!
*smacks forehead* ;-)
Frankly, I think someone needs to have a stern talking-to with the thief that took his images and made their own page out of it, failing to give him credit for his work.
--TwoDragons