So, the size of a black hole is tough to define, as all of a black hole’s mass is concentrated in a single point called the singularity.

However, we can estimate the size of a black hole’s event horizon, which is the region of space that light cannot escape from because of the black hole’s gravity.

If we use this to measure the size, then the largest known black hole would likely be the galaxy in Phoenix A (https://en.wikipedia.org/wiki/Phoenix_Cluster), which would have an event horizon 500 billion miles across.