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A blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet composed of ionized matter traveling at nearly the speed of light) directed towards an observer.
Blazars are powerful sources of emission across the electromagnetic spectrum and are observed to be sources of high-energy gamma ray photons.
Blazars are highly variable sources, often undergoing rapid and dramatic fluctuations in brightness on short timescales (hours to days).
In 2009, a team of astronomers using the Swift spacecraft used the luminosity of S5 0014+81 to measure the mass of its super-massive black hole. They found it to be about 10,000 times more massive than the black hole at the center of our galaxy, or equivalent to 40 billion solar masses
is that its actuall size or the size of its event horizon?
The event horizon, as the singularity inside it is infinitely compressed, and is the smallest something can be, even smaller than a planck length which is the smallest possible measurement.
That's speculation. We don't know the nature of the singularity or even if it is a singularity.
The calculations using general relativity clearly show that past a certain mass the curvature of space time becomes infinitely compressed and so gravity at this point is also infinite.
While we are unable to observe this singularity due to the laws of physics this is not a controversial understanding.
https://www.einstein-online.info/en/spotlight/the-singularity-theorem/
We don't know anything. The point of science is to know what we don't know.
I'm not sure there's a other way to really describe the size of a black hike other than mass or the Schwarzschild radius (the radius to the event horizon.