  Fast radio bursts (FRBs) are micro-to-millisecond duration radio transients
that originate mostly from extragalactic distances. The emission mechanism
responsible for these high luminosity, short duration transients remains
debated. The models are broadly grouped into two classes: physical processes
that occur within close proximity to a central engine; and central engines that
release energy which moves to large radial distances and subsequently interacts
with surrounding media producing radio waves. The expected emission region
sizes are notably different between these two types of models. FRB emission
size constraints can therefore be used to distinguish between these competing
models and inform on the physics responsible. Here we present the measurement
of two mutually coherent scintillation scales in the frequency spectrum of FRB
20221022A: one originating from a scattering screen located within the Milky
Way, and the second originating from a scattering screen located within its
host galaxy or local environment. We use the scattering media as an
astrophysical lens to constrain the size of the lateral emission region,
$R_{\star\mathrm{obs}} \lesssim 3\times10^{4}$ km. We find that this is
inconsistent with the expected emission sizes for the large radial distance
models, and is more naturally explained with an emission process that operates
within or just beyond the magnetosphere of a central compact object. Recently,
FRB 20221022A was found to exhibit an S-shaped polarisation angle swing,
supporting a magnetospheric emission process. The scintillation results
presented in this work independently support this conclusion, while
highlighting scintillation as a useful tool in our understanding of FRB
emission physics and progenitors.
