In order to know what the largest star in the universe is we would have had to look at all the stars. We haven't come close to that - we haven't even looked at all the stars in our galaxy. There are about 100 billion stars in our galaxy, and about as many galaxies in the observable universe so you might understand why.

The best I can do is tell you what the largest known star is, but to do that I also need to know what you mean by largest. Do we pick the brightest, or the largest in volume, or the most massive (the one with the most mass)? More massive stars are generally less dense so take up a proportionally bigger volume. The most massive known is generally accepted to be the Pistol Star, which has a mass about 100-150 times that of the Sun, and is about 10 million times brighter (see this picture). About 100 Suns would provide the same amount of stuff as the star, but we would need more that 100 Suns worth of volume to fill in the space it takes up.

The Pistol Star is still very large in volume. It has a mass about 100 times the mass of the Sun and a radius of about 100 million miles (comparable to the Earth-Sun distance, or about 300 times the radius of the Sun). More massive stars are also less dense so take up proportionally more space. Red giant stars (like Betelgeuse and Antares) are also very big in volume.When our Sun becomes a red giant in about 5 billion years it will expand to enclose the inner planets. Antares has a radius about 400 times that of the Sun, so is bigger than the Pistol star in volume despite being smaller in mass.