  The binding energies, shapes and sizes of even-even beta-stable nuclei with A
>= 40 and a few chains of isotopes with Z=50, 56, 82, 94 protons and isotones
with N=50, 82, 126 neutrons are analyzed. The average isospin dependence of the
radii of protons and neutrons evaluated within the relativistic mean field
theory is studied. A simple, phenomenological formula for neutron radii is
proposed.
