Technically, the Milky Way is a constellation. All the stars of our galaxy form a cloudy band that goes all the way across the sky, and fully around the Earth. My next guess would be the Big Dipper. It's pretty big. Oops, sorry! Hydra The Water Snake. http://www.space.com/scienceastronomy/skywatch.html

The largest of the 88 recognized constellations is Hydra, a long, slender, winding region of stars named after the many-headed sea serpent Hercules encountered in Greek mythology. Stretching from the northern hemisphere, where it borders Cancer, Hydra winds down to the southern sky, where it meets Centaurus. It includes many bright stars, but its total area covers over 1,300 square degrees. It is best seen from the southern hemisphere, but is also visible in the north from January through May. http://www.infoplease.com/askeds/largest-smallest-constellations.html The biggest of all 88 constellations wraps along the Milky Way low in the spring sky and contains three Messier objects. Hydra the Water Snake is tied with the mythology of Crater the Cup and Corvus the Crow. Both Crater and Corvus are perched on the twisting form of the Water Snake. Hydra, largest of all the constellations, fills the space between the important zodiacal constellations along the ecliptic and the Milky Way. Finding the Constellation Hydra Hydra is located low in spring skies for observers in the Northern Hemisphere. The circlet that forms the head of the water snake lies below the constellation Cancer with its famous Beehive Cluster. The body extends away from the head toward the southeast. It runs below the forms of Sextans, Crater, Corvus, Virgo, and all the way to Libra. http://stargazing.suite101.com/article.cfm/observing_hydra_the_water_snake