  The mean-square radii of the molecules $^4$He$_3$, $^4$He$_2-^6$Li,
$^4$He$_2-^7$Li and $^4$He$_2-^{23}$Na are calculated using a three-body model
with contact interactions. They are obtained from a universal scaling function
calculated within a renormalized scheme for three particles interacting through
pairwise Dirac-delta interaction. The root-mean-square distance between two
atoms of mass $m_A$ in a triatomic molecule are estimated to be of de order of
${\cal C}\sqrt{\hbar^2/[m_A(E_3-E_2)]}$, where $E_2$ is the dimer and $E_3$ the
trimer binding energies, and ${\cal C}$ is a constant (varying from $\sim 0.6$
to $\sim 1$) that depends on the ratio between $E_2$ and $E_3$. Considering
previous estimates for the trimer energies, we also predict the sizes of
Rubidium and Sodium trimers in atomic traps.
