  We have performed statistical analysis of a large number of Type III radio
bursts observed by STEREO between May 2007 and February 2013. Only intense,
simple, and isolated cases have been included in our data set. We have focused
on the goniopolarimetric (GP, also referred to as direction-finding) properties
at frequencies between $125$ kHz and $2$ MHz. The apparent source size $\gamma$
is very extended ($\approx60^\circ$) for the lowest analyzed frequencies.
Observed apparent source sizes $\gamma$ expand linearly with a radial distance
from the Sun at frequencies below $1$ MHz. We have shown that Type III radio
bursts statistically propagate in the ecliptic plane. Calculated positions of
radio sources suggest that scattering of the primary beam pattern plays an
important role in the propagation of Type III radio bursts in the IP medium.
