You probably heard they were talking about yanking Pluto off the stage.
Well, today the axe fell. It's not a planet any more. Time to re-learn the planets.
To answer the title of the thread, size doesn't matter so much. In Pluto's case, it was demoted because it didn't clear its orbit of other planets. Namely Neptune. Their orbits overlap. I don't exactly understand why Neptune didn't get demoted for not clearing its orbit of Pluto, but there you go. It's probably the Neptune bias of those French judges, I think.