When designing a Taylor-Couette flow ultraviolet (UV) disinfection system, the proper reactor size is obtained by balancing the diameter and height of the reactor. The disinfection level, which is based on reactor length (short and long reactors) and outside UV sources, is investigated to study the implications of reactor geometries. The experiment was performed by introducing three different sets of photon energy into the reactor from circumferential sources with different lengths. Three sets of equivalent photon energy are delivered to the system by limiting the photon exposed region and the number of sources. In addition, water-soluble caramel is used to establish the photon attenuation. The result indicated that there is no significant difference between long and short reactors with an equivalent UV-exposed region and similar flow rates. Indeed, the result shows that disinfection is strongly affected by the dosage delivered to the system. A short and wider photoreactor may yield similar disinfection levels as a tall and slender device, as long as the dosage delivered and the flow fields are similar. Thus, the reactor height and width can be sized depending on the application and other space limitations. A method is also presented to predict the mean intensity of the photoreactor with outside photon sources.