The ninth planet is believed to be about 4 times the size of the Earth and at about 700 AU (700 times farther from the Sun than the earth is to the sun).
Monthly Notices of the Royal Astronomical Society: Letters - Commensurabilities between ETNOs: a Monte Carlo survey
The data is hinting at two more worlds at 213 and 329 AU (7 and 10 times farther than Pluto). What while they be like? Patryk Lykawka suggested back in 2008 that a Mars-to-Earth mass object (or objects) sculpted the inner Kuiper Belt – the suggested Planets 10 and 11 could well be such.
SOURCE - Crowlspace, Monthly Notices of the Royal Astronomical Society: Letters,