The largest known ‘object’ in the Universe is the Hercules-Corona Borealis Great Wall. This is a ‘galactic filament’, a vast cluster of galaxies bound together by gravity, and it’s estimated to be about 10 billion light-years across! However, this is not really a ‘celestial body’ – a term which normally implies a tightly-bound object like a star or a galaxy.
The largest known elliptical galaxy is thought to be IC 1101 (with a diameter of four million light-years), and the largest known spiral galaxy is Malin 1 (with a diameter of 650,000 light-years). Meanwhile, the largest star by radius is thought to be UY Scuti, a red hypergiant star in the constellation of Scutum that has an estimated radius of over a billion kilometres – 1,700 times that of the Sun.