Spirit Guardians has a range of self and the area of effect is described:
They fly around you at a distance of 15 feet for the duration.
A distance of 15 feet is the same range that you could reach if you have a 15-foot range or a remote weapon with a range of 15 feet. Unless you are using the rule-facing variant, you are expected to have that range in all directions from all the boxes you occupy.
Therefore, the area of effect of Spirit Guardians is the range of 15 feet range of the launcher. Assuming the launcher is small or medium, it is slightly larger than a 15-foot radius because it adds the square in which it stands. like a radius of 15 feet. For a larger or larger creature, the area is much larger.