Most asteroids are in the main asteroid belt between the orbits of Mars and Jupiter. The average temperature on the surface of a typical asteroid is -100 degrees F. Size. Asteroids vary greatly in size, from small boulders to hundreds of miles in diameter.The asteroid belt contains more than 200 asteroids larger than 60 miles wide. Scientists estimate that there are more than 750,000 asteroids in the belt wider than 3/5 mile.There are millions of smaller asteroids. The average distance between asteroids in the main belt is about 600,000 miles.he average temperature on the surface of a typical asteroid is -100 degrees F. Size. Asteroids vary greatly in size, from small boulders to hundreds of miles in diameter. The asteroid belt contains more than 200 asteroids larger than 60 miles wide.