What are the three smallest planets?
Mercury, Venus and Mars are the three smallest. Pluto has been downgraded to a "dwarf planet". In fact it is now believed that it is not a true planet at all according to the official definition; but, because it has been counted among the planets for so long, there was sentiment at retaining it as one. A compromise was made calling it a "dwarf planet".
The biggest planets in size are (from largest down): Jupiter, Saturn, Uranus, Neptune. The smallest major planets are (from smallest up): Mercury, Mars, Venus, Earth. The dwarf planets are smaller than Mercury. Eris is the largest, followed by Pluto, Haimea, and Makemake. The asteroid Ceres is the smallest of the dwarf planets, being only 1/78th the size of Earth's Moon.