Mercury, Venus, Earth, Mars and Pluto are mostly composed of rocks and metals. Jupiter, Saturn, Uranus and Neptune are mostly composed of gas with a small rocky core. They also have rings made of small chunks of ice and rock. Largest asteroid is Ceres, which is about 1000 kilometers across. Most are much smaller, down to the size of dust grains.
Size is about a few kilometers.
This is the Kuiper belt . Other comets are found in the Oort cloud which is a spherical distribution of comets with the orbits more random. We need to explain all observations. The orbits are ellipses but very close to circular, and the spacing between orbits increases with distance from the Sun. The Sun also rotates in the same direction as everything else.