  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.
