Antigen presenting cell (APC) gene delivery is a promising avenue for modulating immunological outcomes toward a desired state. Recently, our group developed a delivery methodology to elicit targeted and elevated levels of APC-mediated gene delivery. During these initial studies, we observed APC-specific structure-function relationships with the vectors used during gene delivery that differ from current non-APC cell lines, thus, emphasizing a need to re-evaluate vector-associated parameters in the context of APC gene transfer. Thus, we describe the synthesis and characterization of a second-generation mannosylated poly(β-amino ester) library stratified by molecular weight. To better understand the APC-specific structure-function relationships governing polymeric gene delivery, the library was systematically characterized by (1) polymer molecular weight, (2) relative mannose content, (3) polyplex biophysical properties, and (4) gene delivery efficacy. In this library, polymers with the lowest molecular weight and highest relative mannose content possessed gene delivery transfection efficiencies as good as or better than commercial controls. Among this group, the most effective polymers formed the smallest polymer-plasmid DNA complexes (∼300 nm) with moderate charge densities (<10 mV). This convergence in polymer structure and polyplex biophysical properties suggests a unique mode of action and provides a framework within which future APC-targeting polymers can be designed.