Here's a breakdown of the sizes and definitions:

* Planet: A celestial body that orbits a star, is massive enough to be rounded by its own gravity, and has cleared its neighborhood of other objects. Planets are the largest objects in our solar system.

* Asteroid: A rocky body that orbits the Sun, typically found in the asteroid belt between Mars and Jupiter. Asteroids are much smaller than planets.

* Comet: An icy body that orbits the Sun. When it gets close to the Sun, the ice vaporizes, creating a visible tail. Comets are typically smaller than asteroids.

* Meteoroid: A small rocky or metallic body in space, typically smaller than an asteroid. When a meteoroid enters Earth's atmosphere and burns up, it's called a meteor (shooting star). If a meteoroid survives its trip through the atmosphere and hits the ground, it's called a meteorite.

So, in terms of size: Planet > Asteroid > Comet > Meteoroid.