In the next picture below, the smallest asteroid shown is Vesta, which is the largest asteroid shown in the above picture. This will give you a sense of scale. Of course the largest object (on the right) is the moon.
Asteroids can be killers. One hit the Earth and wiped out the dinosaurs and blew out a huge hole that became the Gulf of Mexico. So, you say that one large enough to wipe out life on Earth is quite slim and that there aren't that many asteroids out there. Well, think again. Just look how many are out there and most of them are between Mars and Jupiter. Mars is the next planet past the Earth, so they aren't that far away.
Just in cast that you think all asteroids are small rocks, here is an illustration of the four largest asteroids in our solar system compared to the United States.
As you can see, Ceres is the largest followed by Vesta and Pallus and the smallest of the big four is Hygiea. I believe that you can enlarge the above picture by clicking on it so that you can read the descriptions of those four asteroids. Now, let us hope that all those asteroids stay up there where they belong and that they don't go bumping together. That may dislodge some from their orbits and send them hurtling this way. Not that I am astronomical genius or anything, but it makes sense to me that some of them will eventually head for the sun, but until that time, have a great day, you hear?