* Largest by diameter: The star with the largest diameter visible to the naked eye is Betelgeuse in the constellation Orion. It's a red supergiant, and if it were in our solar system, its surface would extend out past the orbit of Jupiter.
* Largest by apparent size: However, Betelgeuse doesn't appear the largest in the sky due to its distance. The star with the largest apparent size in the night sky is R Doradus, a red giant in the constellation Dorado. It's a lot closer to Earth than Betelgeuse, but still quite far away.
So, it depends on how you define "biggest." Betelgeuse is the largest star in terms of diameter, but R Doradus appears the largest in the sky.
