Asteroid images usually look just like star images, unless viewed with extremely high-power or from a spacecraft near the asteroid. Usually, to tell if you are seeing an asteroid instead of a star, you must make several observations to see if the object is moving with respect to the stars in the field of view. They range in size from pebbles (meteoroids) to nearly 1000 km.
This view is from Galileo's historic encounter an asteroid Gaspra on October 29, 1991. The image reveal Gaspra to have a cratered surface. It is an irregularly shaped body about 19 by 12 by 11 kilometers. The spacecraft passed about 1600 kilometers from Gaspra at a relative speed of about 8 kilometers per second.