Colorectal cancer is the fourth leading cause of cancer-related deaths worldwide. Capecitabine is a chemotherapeutic agent commonly used for the treatment of colon cancer. To realize local sustained release, promote efficient local intracellular transport, and mitigate the systemic toxic effects of capecitabine, a capecitabine prodrug, capecitabine-poly (p-dioxanone) (Cap-PPDO), was successfully synthesized. Cap-PPDO was subsequently processed into nanoscale particles with various diameters using electrospraying to investigate the influence of nanoparticle (NP) size on the therapeutic efficiency of Cap-PPDO NPs. Design Expert Software was used to design an experimental scheme for evaluating the influence of electrospraying parameters on NP size and distribution. The in vitro capecitabine release rate of Cap-PPDO NPs was evaluated, and NPs with a size of approximately 300 nm demonstrated the fastest release rate. However, Cap-PPDO NPs with a size of approximately 300 nm exhibited lower proliferation inhibition against SW480 colorectal cancer compared to those with diameters of 200 and 400 nm. To further elucidate the influence of size, the endocytosis of SW480 cells with respect to these differently sized NPs was investigated using flow cytometry and transmission electron microscopy (TEM), given that endocytosis is an important pathway for the intracellular delivery of nanoparticles. The mechanism underlying the size-dependent therapeutic efficiency of Cap-PPDO NPs was ultimately attributed to the size of the mammalian lysosome. Finally, the therapeutic efficacy of Cap-PPDO NPs of various sizes was verified using a nude mouse model of SW480 cell-transplanted tumors.