What do the fusing 'onion layers' of a pre-supernova star look like to scale?
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.