  Low frequency radio wave scattering and refraction can have a dramatic effect
on the observed size and position of radio sources in the solar corona. The
scattering and refraction is thought to be due to fluctuations in electron
density caused by turbulence. Hence, determining the true radio source size can
provide information on the turbulence in coronal plasma. However, the lack of
high spatial resolution radio interferometric observations at low frequencies,
such as with the LOw Frequency ARray (LOFAR), has made it difficult to
determine the true radio source size and level of radio wave scattering. Here
we directly fit the visibilities of a LOFAR observation of a Type IIIb radio
burst with an elliptical Gaussian to determine its source size and position.
This circumvents the need to image the source and then de-convolve LOFAR's
point spread function, which can introduce spurious effects to the source size
and shape. For a burst at 34.76 MHz, we find full width at half maximum (FWHM)
heights along the major and minor axes to be $18.8^\prime$ $\pm~0.1^\prime$ and
$10.2^\prime$ $\pm~0.1^\prime$, respectively, at a plane of sky heliocentric
distance of 1.75 R$_\odot$. Our results suggest that the level of density
fluctuations in the solar corona is the main cause of the scattering of radio
waves, resulting in large source sizes. However, the magnitude of $\varepsilon$
may be smaller than what has been previously derived in observations of radio
wave scattering in tied-array images.
