This is a first report on a novel divergent procedure to synthesize higher generation polyphenylene dendrimers with an Ir(III) core up to G4, which up to now is the largest Ir(III) dendrimer, having a molecular diameter of 8 nm. Our synthetic method provides a much higher yield (>80%) than earlier reported traditional convergent strategies (<35%). Moreover, with a stepwise synthesis, the molecular sizes are controlled by different dendrimer generations from G1 (R-1 approximate to 15 angstrom) to G4 (R-4 approximate to 40 angstrom). In this case, polyphenylene dendrons are used as a "matrix" which prevent iridium phosphorescent cores from triplet-triplet annihilation and improve their photoluminescence quantum yields (PLQYs). All dendrimers show strong phosphorescence at room temperature, and interestingly, their PLQYs tend to increase with subsequent generations up to 36% for G4 in solid state, almost 4 times of that of the nondendritic iridium complex. We also fabricated all generation dendrimers in phosphorescent organic light-emitting diodes (PhOLEDs) and investigated the relationship between the Ir(III) dendrimer sizes and the device performances. Our results indicate that dendrimer G3 possesses the highest efficiency device compared to other generation dendrimers, since its appropriate dendrimer size (R-3 approximate to 30 angstrom) can not only prevent intermolecular triplet-triplet annihilation, thereby increasing the PLQY, but also provide an effective charge carrier mobility from the periphery to the Ir(III) core.