Two-dimensional (2D) pi-conjugated microporous polymers recently attracted tremendous interest due to their unique structural and electronic properties compared with 1D polymers and also graphene. In this paper, we present a comprehensive electronic structure investigation of several representative 2D conjugated polymers by virtue of first-principles density functional theory (DFT) calculations. A comparison of how spatial distribution of frontier molecular orbitals and charge carriers evolves in 1D and 2D conjugated oligomers as a function of system size is given. We also report the relationships between HOMO-LUMO gaps/ionization potential (IP)/electron affinity (EA)/structural reorganization energy upon charge doping and the oligomer size. These findings are insightful for understanding the electronic structure difference between 1D and 2D pi-conjugated polymers and informative for designing new functional materials.