Defining a Dwarf Planet in KSP
We obviously can't go by the IAU definition, there are no objects to clear, there is no real gravity having an effect on anything.
So I propose a definition more relevant to KSP:
A planet that is smaller than the average moon size (212 km) can be considered a dwarf planet.
At the time of this writing, that includes two: Dres and Eeloo. Since it just so happens that these are also the two newest planets, we can likely expect to see more. Therefore, we should begin making the distinction sooner rather than later.