There is no definitive answer to this question as it depends on how you measure galaxy size. For example, if you take into account only the visible stars, then the Andromeda Galaxy is the largest galaxy in the universe. However, if you take into account all of the mass in a galaxy, including dark matter, then the Milky Way is actually larger than Andromeda. There are also different ways to define what a "galaxy" is - some definitions would include satellite galaxies like the Magellanic Clouds, while others would not.
In general, though, most astronomers would agree that IC 1101 is the second largest known galaxy in the universe. IC 1101 is a supergiant elliptical galaxy located in the constellation Serpens Caput. It has a diameter of about 6 million light years and contains an estimated 100 trillion stars - making it more than 1 thousand times more massive than our own Milky Way Galaxy.