LYCOS RETRIEVER 
Asteroids: Size
built 217 days ago
As previously stated, if all the asteroids in the belt were combined into one, it would form a body less than 1500 km in diameter. Noting the immense size of Ceres, it comprises over 1/3 the total suspected mass of the belt (2.3 x 1021 kg).
Source:
Some asteroids such as Mathilde are very light and are probably "rubble piles" made up of lots of small particles loosely held together. Other asteroids are metallic (for example Kleopatra) or pieces of solid rock (Eros, visited by the NEAR spacecraft, is an example) . Sometimes asteroids have small moons or travel in equal-sized pairs. Most asteroids have unusual shapes because they have experienced many collisions and do not have a strong enough gravity to pull themselves back into a sphere. Asteroids are not visible to the unaided eye, but some can be seen with small telescopes or even binoculars.
Source:
[M]any asteroids are probably not homogenous but consist of mixed materials from more than one asteroid, or from more than one part of a large asteroid. When two asteroids collide, much of the debris can fall back together to form an asteroid composed of rubble from the parent body or both bodies in the collision. The degree of dispersal depends mostly upon the relative sizes of the two colliding bodies and their relative velocity.
Source:
Known asteroids range in size from the largest -- Ceres, the first discovered asteroid in 1801 -- at about 600 miles (1,000 kilometers) in diameter down to the size of pebbles. Sixteen asteroids have diameters of 150 miles (240 kilometers) or greater.
Source:
Some asteroids have others revolving around them, just as the Moon revolves around the Earth. Ida, an asteroid about 56km in diameter has its own moon - a tiny body only 1km in size. Other asteroids may well have moons of their own waiting to be discovered.
Source:
MORE ABOUT
Size