At what point is a "solar body" a planet?

I agree with the statement that "a planet is a body that directly orbits a star", but dislike the loose distinction between (real) planets and so-called dwarf planets. Compared to a certain (real) planet called Jupiter, the Earth is relatively small. What is the order of magnitude that makes a solar body a dwarf planet? What is the standard for a full-size (real) planet? What if, in years to come, we discover solar bodies that are incredibly larger or smaller than the adopted standard? Will we end up back in this very spot, searching for a size-based definition of the word "planet"?

I also dislike the concept of a planet as the "dominant object" in its orbit... that just changes the problem from defining "planet" to defining "dominant".

BTW, the planet is now known as Xena, as in the Warrior Princess!