how is asteroid size determined?

Question

My daughter needs this info for a school project and we can't seem to find any site with COPYRIGHTED information to use. If a website is found, it must have a copyright date on it.

Thanks!!

Answer 1

asteroids are determined by calculating their trajectories to get out their weight, along with that their magnitude (visibility) is observed.
With both one can get a coarse idea about an asteroids size.

Another new method is using radar. As far as i remember someone recently took 'pictures' of an asteroid using the arecibo radio-telescope.

Answer 2

via occultation. As the asteroid goes in front of a remote star, they can determine it's size by how long it takes to do so.