The Sun is a sphere. It has a diameter of 32 miles and is located approximately 3000 miles above the surface of the Earth.
The Sun's area of light is limited to a circular area of light upon the earth much like the light of a lighthouse is limited to a finite circular area around it. The rotating light on a lighthouse does not propagate infinitely into the distance. This means that only certain portions of the Earth are lightened at a time. It also describes how night and day arise on a Flat Earth. The apparent view of rising and setting are caused by perspective, just as a flock of birds overhead will descend into the horizon as they fly into the distance.