The optical and electronic properties of quantum dots (QDs), which are drastically affected by their size, have a major impact on their performance in devices such as solar cells. We now report the size-dependent solar cell performance for CuInS2 QDs capped with 1-dodecanethiol. Pyramidal shaped CuInS2 QDs with diameters between 2.9 and 5.3 nm have been synthesized and assembled on mesoscopic TiO2 films by electrophoretic deposition. Time-resolved emission and transient absorption spectroscopy measurements have ascertained the role of internal and surface defects in determining the solar cell performance. An increase in power conversion efficiency (PCE) was observed with the increasing size of QDs, with maximum values of 2.14 and 2.51% for 3.9 and 4.3 nm size particles, respectively. The drop in PCE observed for larger QDs (5.3 nm) is attributed to decreased charge separation following bandgap excitation. Because the origin of photocurrent generation in CuInS2 QDSC arises from the defect-dominated charge carriers, it offers the opportunity to further improve the efficiency by controlling these defect concentrations.