  Motivated by the historical detection of gravitational waves from GW170817,
the neutron star and the neutron drop, i.e., a certain number of neutrons
confined in an external field, are systematically investigated by ab initio
calculations as well as the nonrelativistic and relativistic state-of-art
density functional theories. Strong correlations are found among the neutron
star tidal deformability, the neutron star radius, the root-mean-square radii
of neutron drops, and the symmetry energies of nuclear matter at
supra-saturation densities. From these correlations and the upper limit on the
tidal deformability extracted from GW170817, the neutron star radii, the
neutron drop radii, and the symmetry energy at twice saturation density are
respectively constrained as $R_{1.4M_{\odot}}\leqslant 12.94$ km, $R_{\rm nd}
\leqslant 2.36$ fm, and $E_{\mathrm{sym}}(2\rho_0) \leqslant 53.2$ MeV.
