Fast radio bursts (FRBs) are microsecond-to-millisecond-duration radio transients1 that originate mostly from extragalactic distances. The FRB emission mechanism remains debated, with two main competing classes of models: physical processes that occur within close proximity to a central engine2–4; and relativistic shocks that propagate out to large radial distances5–8. The expected emission-region sizes are notably different between these two types of models9. Here we present the measurement of two mutually coherent scintillation scales in the frequency spectrum of FRB 20221022A10: one originating from a scattering screen located within the Milky Way, and the second originating from its host galaxy or local environment. We use the scattering media as an astrophysical lens to constrain the size of the observed FRB lateral emission region9 to ≲3 × 104 kilometres. This emission size is inconsistent with the expectation for the large-radial-distance models5–8, and is more naturally explained by 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 polarization angle swing10, most likely originating from 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.
