An exhaustively researched (currently in its fifth iteration with future updates including Dune and Mass Effect planned) fictional spaceship size comparison chart by German digital artist Dirk Loechel featuring craft of all descriptions from Star Wars, Warhammer, Babylon5, Star Trek, Wall-E, Serenity, Farscape, Silent Running, Independence Day, AvP, Battlestar Galactica and dozens more.
Link to gob-smackingly massive version here.
UPDATE: Still not sufficiently sated by the rocket-scaling above? Check out Jeff Russel’s Starship Dimensions – an entire site dedicated to this sort of thing.
io9 (H/T: JCDSR)