  We calculate several "size-like" B-8 observables within the microscopic
three-cluster model and study their potential constraints on the zero-energy
astrophysical S_{17}(0) factor of the Be7(p,gamma)B8 reaction. We find within
our three-cluster model that a simultaneous reproduction of the experimental
data for the B-8 radius and quadrupole moment and of the B8-Li8 Coulomb
displacement energy implies S_{17}(0)=(23-25) eVb.
