Sirius is the brightest star in the night sky, but far from the biggest. It is about 1.7 times the radius of our Sun and twice the mass. We see it so brightly because it is very close to us - only about eight light-years away.
The biggest known star is a red hypergiant in the constellation Canis Major known as VY Canis Majoris. This star has a mass between 30-40 times that of our Sun and a radius 1,800 to 2,100 times our Sun's radius. To put that in perspective, if VY Canis Majoris were located at the center of our solar system, the star would be larger than the orbit of Saturn. Despite its size, however, VY Canis Majoris is a bit too faint to be seen with the naked eye due to its distance - 4,900 light years away.