The famous Phillips CrO x /SiO2 catalyst is characterized by its multisite nature and the resulting broad molecular weight distribution (MWD) of its polyethylene (PE) products. However, the multiplicity of active sites and their catalytic behavior relating to the broad MWD have scarcely been studied. Given the high and comparable efficiency of calcination and F modification in regulating the active site multiplicity of this catalyst, eight polyhedral oligomeric silsesquioxane (POSS)-based cluster models comprising four-, six-, and eight-membered chromasiloxane rings (4CR, 6CR, and 8CR) were partially or fully capped by OH or F and employed for density functional theory (DFT) calculations related to C2H4 polymerization. For the OH-capped models, the results emphasized the dominant size effect of the chromasiloxane ring (CR), where the model with a smaller CR was less abundant but could produce higher-molecular-weight (MW) PE with a higher activity. The predicted MW indicated the very broad MWD of the PE products, consistent with that of the products generated from low-temperature calcinated CrO x /SiO2. However, depending on the size of the CR, these models responded unequally to F modification. Specifically, the more abundant sites with 8CR were promoted more obviously, and the MW of their PE products was increased. This makes the polymerization rate higher and the MWD of the PE narrower, which agrees with those of the PE products synthesized by F-modified CrO x /SiO2 calcined at a low temperature. Furthermore, the origin of the effects of CR size and F modification was properly illustrated in terms of both steric and electronic variations.