Hydra is enclosed in a yellow diamond with Nix in orange. The image on the right has been specially processed to reduce the center glare, a result of the over-exposure of Pluto and its tidally-locked (and largest) moon Charon.
Hydra and Nix were only discovered recently (in 2005) and are probably between 25 and 95 miles in diameter. That's a lot of wiggle room, but scientists can't get more precise with the Hubble or other space telescopes -- they need New Horizons to keep saddling up towards Pluto and its moons so the spacecraft can figure out what's what.
There are still at least two more known moons left to see: Styx and Kerberos may both be less than 10 miles across. You can see just how much the dwarf planet dwarfs its tiny moons at the Planetary Society's blog.
New Horizons is expected to make a close encounter with Pluto on July 14.