The reason that Pluto is no longer a planet is not because of its size. In fact, it passes the test for "size" (mass).
The definition of 'planet' was made more strict. It is as following:
1. Massive enough to be round. Very massive bodies have so much gravity that they crush down any irregular edges towards their centre, and so become ball-like.
2. The primary object orbiting the Sun. For instance, the Moon orbit's the Sun, but it does so by orbiting the Earth. The Earth is the primary object orbiting the Sun.
3. Has cleared it's own orbit. Planets clear their orbits of debris/asteroids (by attracting them with their gravity).
Pluto passes 1 and 2, but has not passed 3. It has not cleared it's orbit of debris.
Objects like Pluto are called Dwarf Planets.
