* Planets: Planets are large celestial bodies that orbit a star. They are significantly larger than asteroids.

* Comets: Comets are icy bodies that orbit the sun. While they can be quite large, they are typically smaller than planets and moons.

* Moons: Moons are natural satellites that orbit planets. They can vary in size, but are generally larger than asteroids.

* Asteroids: Asteroids are rocky bodies that orbit the sun, primarily found in the asteroid belt between Mars and Jupiter. They are generally much smaller than planets, moons, and even some comets.

Therefore, asteroids are usually the smallest of the objects listed.