I use a method based on interstellar scintillations for discerning information about source sizes on scales less than one micro-arc sec .
I use a comprehensive model for a pulsar signal , scintillated amplitude modulated noise , that includes source fluctuations and noise statistics .
The method takes into account time-frequency averaging in the signal processing as well as effects due to source structure .
The method is applied to interferometric visibility data on the Vela pulsar which show slightly less scintillation modulation than expected for a point source in the strong scattering regime .
The decreased scintillation modulation is likely to be due exclusively to time-frequency averaging rather than from any source size effects .
The implied upper limit on source extent , derived through Bayesian inference , is compared to predictions that take into account beaming from the relativistic plasma flow in neutron star magnetospheres .
The upper limit for the transverse source size ( \lesssim 400 km at 95 % confidence for a pulsar distance of 0.5 kpc ) is easily consistent with conventional models for radio emission regions in pulsar magnetospheres that place them well inside the light cylinder at only a few neutron-star radii from the star ’ s surface .