We calculate several “ size-like ” ^ { 8 } B observables within the microscopic three-cluster model and study their potential constraints on the zero-energy astrophysical S _ { 17 } ( 0 ) factor of the ^ { 7 } { Be } ( p, \gamma ) { { } ^ { 8 } { B } } reaction .
We find within our three-cluster model that a simultaneous reproduction of the experimental data for the ^ { 8 } B radius and quadrupole moment and of the ^ { 8 } B- ^ { 8 } Li Coulomb displacement energy implies S _ { 17 } ( 0 ) = ( 23 - 25 ) eV b .