Diameter Is 750 Miles. Charon Is The Biggest Of Pluto's Known Moons.

Styx's Radius Is Unknown. Styx Seems To Be The Smallest Of Pluto's Moons.

Nix Is 35 X 16 Miles.

Charon

Styx and Nix

Kerberos and Hydra

Pluto's Moons

Kerberos' Diameter Is 18 Miles.

Hydra Is 36 X 21 Miles!

http://www.nasa.gov/content/goddard/briefing-materials-observations-of-pluto-s-moons