Up to two-thirds the size of Earth is not “Earth-sized”
Predicted? Meaning they don’t really have any evidence
Who sez up to 2/3rds isn’t Earth-sized, you? Big deal.
The evidence for an undiscovered outer solar system body consists of comet focusing (Matese et al); the large (Pluto-sized) bodies on very long, eccentric orbits (Brown et al); and the people in this article based their figures on a computer simulation based on retrocalculating the orbits of known bodies; others.
The perturbation of Neptune, detected not many years after discovery and the calculation of its ephemeris, has been attributed by some to a few bad observations, but that’s a minority view. My own minority view is, the perturbation was brief because the planetary body is either A) orbiting more or less in the ecliptic, but in retrograde, so everyone looking for it winds up looking in the wrong direction, or B) orbiting out of the ecliptic, and perhaps also in retrograde.
Then the article says between 30% to 70% the size of the Earth, so that would mean half the size of Earth if the theory is correct.