pi-Linked organic small molecular acceptor materials (also known as A-pi-A quasi-macromolecule (QM) acceptors) have garnered significant attention in organic solar cells (OSCs) due to their well-defined structures, reproducibility, improved morphology, and enhanced stability. Altering the pi bridge unit is a simple yet effective method to modulate molecular configuration and packing motifs, which in turn affects the efficiency of the resulting OSCs. Herein, we synthesized three A-pi-A QM acceptors, QM-1T, QM-2T and QM-3T, with varying conjugation lengths of the pi bridge units (thiophene, bithiophene and terthiophene) and explored the exquisite control of molecular size to influence the active layer morphology and device performance. Theoretical calculations and experimental characterization results demonstrate that QM-2T exhibits increased absorption, an upshifted LUMO level, and a more ordered stacking pattern due to its relatively suitable pi bridge length. The well-controlled morphology in the PM6:QM-2T blend also results in the much-improved and balanced electron and hole mobility. Consequently, QM-2T-based OSC achieves a high open circuit voltage of 0.94 V without sacrificing short circuit current density, resulting in a higher device efficiency of 17.86% compared to QM-1T and QM-3T. These findings underscore the importance of molecular geometric design by featuring conjugated pi bridge lengths to achieve high-performance OSCs.