Compounds of the formula (EDO-TTF-Cl)2XF6 (EDO-TTF-Cl = 4-chloro-4′,5′-ethylenedioxytetrathiafulvalene, X = As, Sb) were prepared to examine the substituent size effect on the packing structure of the donor molecules. In these salts, a head-to-tail type donor stacking was observed. Although the AsF6 salt is a quasi-one-dimensional dimer Mott insulator, the SbF6 salt shows quasi-one-dimensional metallic behavior. Detailed crystal structure analyses revealed the correlation between the anion size and the intermolecular slipping among the donor molecules. The donor morphology, which plays an important role in the overlapping mode selectivity, is characterized by the magnitude and configuration of the out-of-plane substituent size.