  A systematic study of the root-mean-square distance between the constituents
of weakly-bound nuclei consisting of two halo neutrons and a core is performed
using a renormalized zero-range model. The radii are obtained from a universal
scaling function that depends on the mass ratio of the neutron and the core, as
well as on the nature of the subsystems, bound or virtual. Our calculations are
qualitatively consistent with recent data for the neutron-neutron
root-mean-square distance in the halo of $^{11}$Li and $^{14}$Be nuclei.
