  We performed an analysis of the main theoretical uncertainties that affect
the radius of low- and very-low mass-stars predicted by current stellar models.
We focused on stars in the mass range 0.1-1Msun, on both the zero-age
main-sequence (ZAMS) and on 1, 2 and 5 Gyr isochrones. First, we quantified the
impact on the radius of the uncertainty of several quantities, namely the
equation of state, radiative opacity, atmospheric models, convection efficiency
and initial chemical composition. Then, we computed the cumulative radius error
stripe obtained by adding the radius variation due to all the analysed
quantities. As a general trend, the radius uncertainty increases with the
stellar mass. For ZAMS structures the cumulative error stripe of very-low mass
stars is about $\pm 2$ and $\pm 3$ percent, while at larger masses it increases
up to $\pm 4$ and $\pm 5$ percent. The radius uncertainty gets larger and age
dependent if isochrones are considered, reaching for $M\sim 1$Msun about
$+12(-15)$ percent at an age of 5 Gyr. We also investigated the radius
uncertainty at a fixed luminosity. In this case, the cumulative error stripe is
the same for both ZAMS and isochrone models and it ranges from about $\pm 4$
percent to $+7$ and $+9$($-5$) percent. We also showed that the sole
uncertainty on the chemical composition plays an important role in determining
the radius error stripe, producing a radius variation that ranges between about
$\pm 1$ and $\pm 2$ percent on ZAMS models with fixed mass and about $\pm 3$
and $\pm 5$ percent at a fixed luminosity.
