I will give you a brief summary.
From ancient times the known planets (wanderers) were lights in the sky which 'wandered' around relative to the fixed stars. These were Mercury, Venus, Mars, Jupiter and Saturn. With the invention of the telescope, two more planets were found? Uranus (1781) and Neptune (1846). Astronomers still looked for new objects (planets, comets, etc.) and in 1930 Pluto was discovered. It turned out to be different.
Mercury, Venus, Earth and Mars are comparatively small and have lost virtually all the hydrogen and helium they once had as they are comparatively close to the Sun. The next four planets are Jupiter, Saturn, Uranus and Neptune. These are much further from the Sun where it is much colder and they have kept large amounts of gases. After Pluto was found it was found to be small and rocky and also to have an orbit at angles to the others (which are all similar).
Nevertheless, Pluto was recognized as a planet. As our means improved, larger telescopes, telescopes in space and missions to distant parts of the Solar System, more small objects were found, starting with Eris (2005). Eris is almost the same size as Pluto. Haumea (discovered 2004/5) is only one third the sie of Pluto - should this also be called a planet, along with Eris? Makemake was also found in 2005, and is about two thirds the size of Pluto. When would this end? (It is still going on). Should these smaller lumps of rock all qualify as planets? In 2006 the IAU decided to name a new category of object - the dwarf planet. Pluto was downgraded to this category as were some of the newly found objects.
The choice was to call all these objects planets or to create a new category. They chose the latter.
Here is a list of some smaller objects:
Those called Trans Neptunian Objects (TNOs) include these smaller bodies further out than Neptune.
For comparison, here is a list of some bodies in the Solar System, together with their mean radii (km):
Jupiter 69 910 Gas giant
Saturn 58 230 Gas giant
Uranus 25 360 Ice giant
Neptune 24 620 Ice giant
Earth 6 371 Rocky planet
Venus 6 052 Rocky planet
Mars 3 390 Rocky planet
Ganymede 2 634 Moon of Jupiter
Titan 2 575 Moon of Saturn
Mercury 2 440 Rocky PLANET
Callisto 2 403 Moon of Jupiter
Io 1 821 Moon of Jupiter
Moon 1 738 Moon of Earth
Europa 1565 Moon of Jupiter
Triton 1353 Moon of Neptune
Eris 1200 TNO
Pluto 1137 TNO
Haumea 1218 TNO
Titania 789 Moon of Uranus
You may find estimates vary from source to source; this list gives the 19 largest bodies (after Sun) in the Solar System
As you see, Pluto is smaller than 4 moons of Jupiter, the largest moons of Saturn and Neptune, and smaller than our own Moon, and about half the size of Mercury.