  The Sun emits radiation at several wavelengths of the electromagnetic
spectrum. In the optical band, the solar radius is 695,700 km and this is what
defines the photosphere, the visible surface of the Sun. However, as the
altitude increases, the electromagnetic radiation is produced at other
frequencies, causing the solar radius to change as function of wavelength.
These measurements enable a better understanding of the solar atmosphere and
the radius dependence on the solar cycle is a good indicator of the changes
that occur in the atmospheric structure. We measure the solar radius at the
subterahertz frequencies of 0.212 and 0.405 THz -- i.e., the altitude where
these emissions are primarily generated -- and also analyse the radius
variation over the 11-year solar activity cycle. For this, we used radio maps
of the solar disk for the period between 1999 and 2017, reconstructed from
daily scans made by the Solar Submillimeter-wave Telescope (SST), installed at
El Leoncito Astronomical Complex (CASLEO) in the Argentinean Andes. Our
measurements yield radii of 966.5" $\pm$ 2.8" for 0.2 THz and 966.5" $\pm$ 2.7"
for 0.4 THz. This implies a height of $(5.0 \pm 2.0 \times 10^6)$ m above the
photosphere. Furthermore, we also observed strong anti-correlation between the
radius variation and the solar activity at both frequencies.
