Pluto is a dwarf planet. It's not considered a "big" or "small" planet, because it doesn't meet the criteria for a planet:

* Orbits the sun: Pluto does this.

* Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape: Pluto is roughly round.

* Has cleared the neighborhood around its orbit: This is where Pluto falls short. It shares its orbital space with other objects in the Kuiper Belt, a region of icy objects beyond Neptune.

So, while Pluto is bigger than many asteroids and comets, it's smaller than the eight "true" planets in our solar system.