Mission scientists have found Pluto to be 2,370 kms in diameter, somewhat larger than many prior estimates.
Images acquired with the Long Range Reconnaissance Imager (LORRI) aborad New Horizon probe were used to make this determination.
The result confirms that Pluto is larger than all other known solar system objects beyond the orbit of Neptune, the US space agency said in a statement.
"The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest," said mission scientist Bill McKinnon from Washington University in St Louis.
Pluto's newly estimated size means that its density is slightly lower than previously thought, and the fraction of ice in its interior is slightly higher.
Also, the lowest layer of Pluto's atmosphere, called the troposphere, is shallower than previously believed.
Measuring Pluto's size has been a decades-long challenge due to complicating factors from its atmosphere.
Its largest moon Charon lacks a substantial atmosphere, and its diameter was easier to determine using ground-based telescopes.
New Horizons observations of Charon confirm its previous estimated size of 1,208 kms across.
Two other moons - Nix and Hydra - were discovered using the Hubble Space Telescope in 2005.
Nix is estimated to be about 35 kms across while Hydra is roughly 45 kms across.
These sizes lead mission scientists to conclude that their surfaces are quite bright, possibly due to the presence of ice.
Pluto's two smallest moons, Kerberos and Styx, are smaller and fainter than Nix and Hydra and are harder to measure.
Mission scientists believe that should be able to determine their sizes with observations New Horizons will make during the flyby and will transmit to Earth at a later date.