  The Sun is an extraordinary workbench, from which several fundamental
astronomical parameters can be measured with high precision. Among these
parameters, the solar radius $R_{\odot}$ plays an important role in several
aspects, such as in evolutionary models. Despite the efforts in obtaining
accurate measurements of $R_{\odot}$, the subject is still debated and
measurements are puzzling and/or lacking in many frequency ranges. We aimed to
determine the mean, equatorial, and polar radii of the Sun ($R_c$, $R_{eq}$,
and $R_{pol}$) in the frequency range 18.1 - 26.1 GHz. We employed single-dish
observations from the newly-appointed Medicina "Gavril Grueff" Radio Telescope
and the Sardinia Radio Telescope (SRT) throughout 5 years, from 2018 to
mid-2023, in the framework of the SunDish project for solar monitoring. Two
methods to calculate the radius at radio frequencies are considered and
compared. To assess the quality of our radius determinations, we also analysed
the possible degrading effects of the antenna beam pattern on our solar maps,
using two 2D-models. We carried out a correlation analysis with the evolution
of the solar cycle through the calculation of Pearson's correlation coefficient
$\rho$. We obtained several values for the solar radius - ranging between 959
and 994 arcsec - and $\rho$, with typical errors of a few arcsec. Our
$R_{\odot}$ measurements, consistent with values reported in literature,
suggest a weak prolatness of the solar limb ($R_{eq}$ > $R_{pol}$), although
$R_{eq}$ and $R_{pol}$ are statistically compatible within 3$\sigma$ errors.
The correlation analysis using the solar images from Grueff shows (1) a
positive correlation between the solar activity and the temporal variation of
$R_c$ (and $R_{eq}$) at all observing frequencies, and (2) a weak
anti-correlation between the temporal variation of $R_{pol}$ and the solar
activity at 25.8 GHz.
