The largest star I've ever found, with a diameter of 162 AU!
That would be 81,000 light-seconds. Which means it would take light 22-and-a-half hours to get from one end of the star to another, if the mass of the star wasn't in the way. Because it takes light 20,000 years to climb out of the center of our sun and into space where it zips to Earth.
I think there are some physic laws which would make this kind of star collapse on itself. On top of gravity, of course (this kind of monster would become a black hole in no time)