[MichaelApproved]

I believe the singularity point, where all the mass collects, is small (dimensionless) but the gravity well that is created by the mass is what is measured when the size is determined. The edge of the gravity well is referred to as the event horizon. The event horizon of a black hole is the point which matter can no longer escape the gravity of the black hole.

However, I've been wrong several times already in this thread so take it for what that's worth.