What do the fusing 'onion layers' of a pre-supernova star look like to scale? I'm sure we've all seen the diagrams of various layers of element fusion from hydrogen to silicon in a star that's just about to go supernova.


(Picture from courses.lumenlearning.com)

I suspect these pictures grossly exaggerate the relative radii at which these fusing layers occur for the sake of readability. What would a more accurate to-scale picture of these fusing layers look like?

Now, I assume the answer will vary significantly based on the mass of the star, and I suspect in certain mass regimes some layers will not be fusing at all. I can think of a few other variables that may affect the answer too.

So in CYA fashion, I'll leave it up to the answerer to identify specific illustrative or interesting cases, as I'm not after a specific answer but rather a general feel for how large some of the layers are in comparison to the others.
Pre- supernova models often characterise the compactness of the core using a "compactness parameter" defined as
$$ \upsilon = \frac{(M/M_{\odot})}{R(M)/1000\ {\rm km}},$$
where $M$ is usually chosen to be $2.5M_{\odot}$ and $R(M)$ is the radius within which $M$ is contained.

Pre supernova models by Farmer et al. (2016) show that the central $2.5M_{\odot}$ of a massive star included the Carbon-burning core in a $15M_{\odot}$ (initial mass) star, but only contained the oxygen-burning core in a more massive star (models are presented up to 30$M_{\odot}$. i.e. Your answer is going to be mass and composition dependent (these models are for a solar metallicity initial composition).

In the $15M_{\odot}$ model (with mass-loss), $\upsilon \sim 0.08$ at core collapse, which means the carbon burning core would have been contained within 31,000 km. The $30M_{\odot}$ model is more compact with $\upsilon= 0.58$, indicating that the oxygen-burning core would have been contained within 4,300 km.

At collapse the iron core might have a mass of 1.4-1.8$M_{\odot}$, will be supported by electron degeneracy pressure and should have a size a bit smaller than a typical carbon white dwarf (radius of a few thousand km).

You can compare these sizes with the size of the entire red supergiant star, which might have a radius of a few au (e.g. Betelgeuse).

The shell-burning regions would be found out to slightly larger radii than this I think, but these numbers are a reasonable estimate. Looking in detail at the models I don't think that the final stages of pre- supernova evolution look anything like the strictly stratified onion-shell picture that is seen all over the internet.