Catalyzes the condensation of three molecules of isopentenyl diphosphate with farnesyl diphosphate (FPP) to yield (all-E)-hexaprenyl diphosphate (HexPP; C30), the precursor of the prenyl side chain of menaquinone-6. Large subunit Hexs-B catalyzes the condensation reaction and the final product chain length is cooperatively regulated by both the Hexs-A and Hexs-B subunits using the whole size of the hydrophobic cleft as a ruler. (2E,6E)-farnesyl diphosphate + 3 isopentenyl diphosphate = all-trans-hexaprenyl diphosphate + 3 diphosphate Binds 2 Mg(2+) ions per subunit. Dimer of heterodimer or heterotetramer composed of a small (Hexs-a) and large (Hexs-B) subunit. Belongs to the FPP/GGPP synthase family.