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I don't think you should infer the radius of the star from the blacked out region. I think they just do that because the luminosity of the star is so intense it would blow away the sensitivity needed to see the planets. So they set everything to zero for a certain zone on the lens/sensor. It's not the physical surface of the star.

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There are a handful of known red supergiant stars around 15 AU in diameter, 20 AU would be pushing past the boundaries of what we believe would be possible.

The star in the video, HR 8799, is about 50% larger than the sun.

Good call!

It looks like HR8799 that the animation is based on is about 1.5x the radius of the sun.

That being said, my silly point still holds - there are stars that are truly massive. Stephenson 2-18 is approximately the size I mentioned, which is genuinely impossible to comprehend.

It's harder to comprehend smallness. The journey to planc constant is longer than the size of our observable universe.

Yeah, but remember that those red supergiants' outer envelope is extremely sparse. The density is compared to the very upper layers of Earth's atmosphere - basically vacuum. Around 90% of a supergiants' radius is taken up by that ultra-thin gas envelope.

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