In the case of a black hole mass and size (radius) are inextricably and linearly linked. All black holes with a given mass have the same radius (modulo a small effect of angular momentum).

Mass is the only really meaningful measure of a blackhole, as they occupy a spacial singularity, which is to say, the have no measurable size.

The event horizon has a diameter, but that's not the object itself.

Guessing that "size" for black holes can mean mass, volume, or radius. But I'm pretty sure the relationship between those three quantities is well defined and directly proportional, so any way you look at it, this thing is incomprehensibly huge.

I believe for black holes the volumetric size is a known function of mass. If you exclude the rest of any Lyman-alpha blob present around the black hole.