The asteroids are mostly concentrated in specific places of the solar system, as the known asteroid belt, the Kuiper belt (farther away from Neptune’s orbit), and the Oort cloud, which contains thousands of millions of comets and asteroids that, some of them, sometimes approach to the Sun and the Earth. Even if all of them are very similar in their composition, the asteroid’s size varies a lot.
The biggest of them
There are only very few asteroids of big dimensions. In fact, the biggest of them is a minority. In the asteroid belt, the biggest asteroids are the named Pallas, Vesta, and Juno, of approximately 500 kilometres each of them. Even if they have a big size, these ones aren’t the biggest objects that we find in the asteroid belt. In this belt, there is also the dwarf planet Ceres, which in the beginnings, and during many time, it was considered as an asteroid. However, it was ascended to the category of dwarf planet in 2006, for its size of 950 kilometres in diameter. However, as isn’t considered an asteroid, Pallas, Vesta, and Juno are therefore the biggest asteroids in the asteroid belt.
Vesta is also the unique body of the belt that, on some occasions, it can be seen at first sight from the Earth. In the same way, we can sometimes also see at first sight nearer asteroids to the Earth, as Apophis, that the 21st of May 2021 passed at only 2 million kilometres from the Earth (en distances of the universe, 2 million kilometres is nothing).
The rest of asteroids
While only some tens of asteroids possess big sizes, of kilometres or thousands of kilometres, the big majority of them have very small sizes. The majority of bodies in the asteroid belt have a size of millimetres, and are fragments of rocks and dust. These ones are the named micro-meteoroids, which are the big majority of the bodies that form the asteroid belt.
Nevertheless, there are also many asteroids that have a size of metres and tens of metres. Even if these ones can seem harmless, an asteroid of 25 metres of diameter that impacted the Earth would release an energy of a megaton (the bomb that was thrown on Hiroshima released an energy of 0,015 megatons).
The smallest asteroids have an irregular form, while the biggest of them, as Vesta or Juno, have a more spherical shape. This more spherical shape is the consequence of the bigger gravity that have the bigger bodies.