  We explore the description of the bound $^{19}$B isotope in terms of a
$^{17}$B+n+n three-body system where the two-body subsystems $^{17}$B+n and
neutron-neutron (nn) have virtual states close to the continuum. Dimensionless
scaling functions for the root-mean-square (rms) radii are defined and studied
for different parameters of the neutron-core potential and considering three
different models for neutron-neutron interaction. The scaling functions for the
radii are rooted on the universal behavior of three-body systems close to the
Efimov limit and depend only on dimensionless quantities formed by the
two-neutron separation energies and scattering lengths. Our results show in
practice the model independence of these scaling functions close to unitarity.
We provide an estimation of the different rms relative separation distances
between the constituents, as well as of the proton and matter radii.
