The biggest known star, in terms of size, is UY Scuti. UY Scuti is a red supergiant star located in the constellation Scutum, approximately 9,500 light-years away from Earth. Its exact size is still uncertain, but estimates suggest that it has a radius of about 1,700 times that of the Sun. If UY Scuti were placed in the center of our solar system, its outer layers would extend beyond the orbit of Jupiter.
In terms of mass, the star R136a1, located in the Large Magellanic Cloud, holds the record for being one of the most massive stars discovered. Estimates suggest that it has a mass about 265 times that of the Sun. However, it's worth noting that R136a1 is a relatively young star and is in a different stage of its evolution compared to UY Scuti. While R136a1 is more massive, UY Scuti is larger in terms of physical size.
It's important to keep in mind that our understanding of the universe is continually evolving, and new discoveries could potentially reveal even more massive or larger stars in the future.