The largest star that is known is a star in the constellation of Auriga, the charioteer. Its name is Epsilon Aurigae. It is thought to be a gigantic star that is about 2,700 times the size of the Sun.

Another answer I found is:

The largest known star (in terms of mass and brightness) is called the Pistol Star. It is believed to be 100 times as massive as our Sun, and 10,000,000 times as bright.