Photosynthetic organisms can achieve near perfect quantum efficiency under optimal condition. This is remarkable considering that electronic excitations should travel about 100 nm or larger distances through rugged, fragile, and dynamic membrane protein environments. There are various structural motifs and arrangements of pigment-protein complexes adopted by different organisms, but the exact molecular and organizational factors enabling their superb capability to relay excitation energy are poorly understood at present. This is true even for one of the most well characterized systems, the photosynthetic unit (PSU) of purple bacteria, which has only two types of antenna complexes called light harvesting 1 (LH1) and light harvesting 2 (LH2) with known crystal structures. Of these, the LH2 serves as the major initiator and carrier of the excitation energy. This talk reports comprehensive series of classical simulations and quantum calculations aimed at understanding the relationship between natural sizes of LH2s and their functionality, while considering realistic molecular details and the effects of disorder. The analysis of computational results leads to a convincing scenario that natural sizes of LH2s are outcomes of the interplay between structural constraints of hydrogen bonding and the need to optimize their energy transfer capability against the disorder.