I am talking about stuff like what happens towards the end of this gif.

Basically big ol' fireballs from stuff like this from the Trinity Test are a result of the enormous amount of energy released by the detonation interacting with the atmosphere.

Without an atmosphere there is no medium to create the fireball. So due the fact that there is no scientific reason for their being giant fireballs in like first gif it is impossible to quantify exactly what the fireball means for the yield of the weapon/attack.

So in general what would a large super energetic explosion look like?

A: If the energy is released in considerably less than a second it would look like a flash bulb. Just a sudden flash of light then nothing.

B: If the energy is released over longer periods it would look like more like an illumination round (minus the smoke trail of course).

There is one exception to this and one semi-exception to this.

The first is if the weapon is using something like degenerate matter as its warhead. Basically the super dense material can create a zone around the explosion which reachs densities equal to or greater than the earth's atmosphere while also being super energetic (and therefor bright). So something like that might produce something that looks like a fireball.

The semi-exception is like this where we can see that the "fireball" is doing some kind of work. In this case it would theoretically be possible to figure out how much j/m² the "fireball" is putting out at a given distance from the epicenter based on it kicking around (in this case) the stuff that makes up Saturn's rings. Once you have the intensity (j/m²) and distance you can work backwards with the inverse square law to come up with a number to get a general idea how strong the blast might be. But that is less "look at my giant fireball" and more "look at my omnidirectional explosion apply work to things that are a long distance away".