  Neutron star radii are primarily determined by the pressure of isospin
asymmetric matter which is proportional to the slope of the nuclear symmetry
energy. Available terrestrial laboratory data on the isospin diffusion in
heavy-ion reactions at intermediate energies constrain the slope of the
symmetry energy. Using this constraint, we show that the radius (radiation
radius) of a 1.4 solar mass neutron star is between 11.5 (14.4) and 13.6 (16.3)
km.
