Planet X is supposed to be Neptune sized. This thing is less than 1000 miles across. 'Giant' for a Kupier belt object; not so much for planets.

Doubt it, it's too small (even the upper end of the estimated size range has it smaller than Pluto). Batygin and Brown's predicted planet would have to be bigger than Earth.

With their current use of terminology a regular-sized planet would be a small medium at large.