Various two-dimension (2D) side-chain substituted benzo(1,2-b:4,5-b’)dithiophene (BDT) block have been used to construct donor polymers, while the size effect of side-chains on photovoltaic performance was overlooked in past few years. This work, three size varied conjugated space (benzene, naphthalene and biphenyl) were introduced into corresponding polymers PBDB-Ph, PBDB-Na and PBDB-BPh. This space engineering has significant impact on the extent of phase separation in active layer which blended with polymer and acceptor ITCPTC and preserved the desired morphology. Varied space size in side-chains leading to distinct balance mobility ratios of hole to electron (benzene is 0.21, and for naphthalene is 0.75, biphenyl is 0.57). Finally, PBDB-Na-based polymer solar cells (PSCs) deliver the highest power conversion efficiency (PCE) 12.52% when compared to the PSCs performance of PBDB-Ph (8.48%) and PBDB-BPh (11.35%). The mothed of tailoring the side-chains structures for fabricating balance between phase separation and charge transport would provide enlightenment for the development of photovoltaic device.