Conjugated microporous polymers (CMPs) have been demonstrated as promising heterogeneous photocatalysts due to its high surface area and adjustable redox potential. Unfortunately, CMPs, which are usually micrometer-scale particles, have limited substrate accessibility to photo-generated excitons or free carriers because of short light penetration depths and hindered mass transfer through the materials. Given that exciton diffusion length in typical conjugated polymers is 5 to 20 nm, only thin surface layer of the CMP particles can effectively work as photocatalysts. This inefficient use of CMPs highlights the need for well-dispersed CMP nanoparticles (NPs) so that all parts of CMPs can contribute to photocatalysis and achieve greater photocatalytic efficiency. To address this need, we recently synthesized hairy CMP NPs via mini-emulsion cross-coupling reactions and following surface-initiated RAFT polymerization, and demonstrated their higher photocatalytic reactivity by enhanced substrate accessibility. However, the average size of the core CMP NPs (~150 nm) was huge compared to the exciton diffusion length, implying there is still an opportunity to further improve the performance. Here, we present a convenient one-step approach to furnish hairy CMP NPs with various core size (10 nm to ~150 nm) using dispersion polymerization. This method is based on single phase cross-coupling reactions of CMP monomers with a “Designer Unit”, where its polymer part sterically stabilizes CMP NPs during the reaction, determining their final size and shape. By varying the concentrations of the Designer Unit and its polymer chain length, we created 16 different hairy CMP NPs and investigated the impacts of core size, surface area, optical and electrochemical properties on their photocatalytic reaction rates. Among these parameters, the core size instead of surface area turns out to be the most important parameter that governs the photocatalytic reactivity; the smaller, the better.