A black hole is a point. Its size is zero, regardless of its mass. Its event horizon has a size, but we don't need to care about that. We do, however, need to

A black hole is a point. Its size is zero, regardless of its mass. Its event horizon has a size, but we don't need to care about that. We do, however, need to care about the all the stuff that's revolving around that point, because even a slight imbalance will cause the galactic center of mass (barycenter) to move away from the black hole. Things get pretty wobbly out there.