Steins is a main belt asteroid in orbit around the Sun between Mars and Jupiter. Rosettas closest distance to Steins was 800 kilometers. Steins is about 5.9 by 4 kilometers large – mountain sized.
Emily Lakdawalla at The Planetary Society has updated her montage that compares the sizes of comets and asteroids that have been imaged. Here you can see the relative size of Steins to other objects imaged by space probes.