What does it take to be a dwarf planet?
The 2006 IAU definition of a dwarf planet states that such a body should be massive enough to achieve hydrostatic equilibrium, it must orbit the sun, but it cannot "clear its own orbit." In the latter case, planets (like Earth) are able to fulfill this criterion as they have a stronger gravitational pull, "sweeping" their orbit clear of debris. Dwarf planets do not have this kind of gravitational oomph, so their orbits are often filled with asteroids, other dwarf planets and even planets.
Considering all the criteria for dwarf planets, plus the rule that they must be of certain brightness, dwarf planets have a minimum radius of 420 km (260 miles). But the Australian astronomers want this minimum size to be decreased, down to 200 km (124 miles) for icy bodies and 300 km (186 miles) for rocky bodies.
Why? Well, this is the limit at which small solar system bodies go from being "potato-like" to a more dwarf planet-looking spherical shape. It is therefore dubbed the ‘potato-limit.'
The nicest thing about this proposal is that it sets a physical size of what it takes to be a small dwarf planet. Until now, the IAU definition has been rather arbitrary. But that's science for you, continually developing and refining our understanding of the celestial zoo on our interplanetary doorstep.