  Measurements of the radius and limb brightening of the Sun provide important
information about the solar atmosphere structure and temperature. The solar
radius increases as the observation at radio frequency decreases, indicating
that each emission originates higher in the atmosphere. Thus, different layers
of the solar atmosphere can be probed by observing at multiple wavelengths. In
this work, we determined the average radius and limb brightening at 100, 212,
230, and 405 GHz, using data from the Solar Submillimeter Telescope and ALMA's
single-dish observations. For the first time, limb brightening values for
frequencies of 212 and 405 GHz were estimated. At sub-THz frequencies, the
observed limb brightening may affect the solar radius measurements. We use two
different and well known approaches to determine the radius: the half-power
method and the inflection-point method. We investigate how the antenna beam
size and the limb brightening level, LB, can affect the radius measurements
using both methods. Our results showed that the inflection-point method is the
least affected by these parameters, and should thus be used for solar radius
estimates at radio wavelengths. The measured average radii are 968"~$\pm$~3"
(100 GHz), 963"~$\pm$~3" (212 GHz), 963"~$\pm$~2" (230 GHz), and 963"~$\pm$~5"
(405 GHz). Finally, we used forward modeling to estimate the ranges of LB of
the solar disk resulting in 5%-19% (100 GHz), 2%-12% (212 GHz), 6%-18% (230
GHz), and 3%-17% (405 GHz). Both radius and limb brightening estimates agree
with previous measurements reported in the literature.
