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Photodetectors based on colloidal quantum dots (QD)/graphene nanohybrids are quantum sensors due to strong quantum confinement in both QD and graphene. The optoelectronic properties of QD/graphene nanohybrids are affected by the quantum physics that predicts a high photoconductive gain and hence photoresponsivity (R*) depending on the pixel length (L) as R*∝L−2. Experimental confirmation of the effect of the pixel geometric parameters on the optoelectronic properties of the QD/graphene photodetector is therefore important to elucidate the underlying quantum physics. Motivated by this, an array of PbS QDs/graphene nanohybrid photodetectors are designed with variable QD/graphene pixel length L and width (W) in the range of 10–150 µm for a study of R*, noise, and specific detectivity (D*) in a broad spectrum of 400–1500 nm. Intriguingly, R* exhibits a monotonic decreasing trend of 1/L2 while being independent of W, confirming experimentally the theoretical prediction. Interestingly, this geometric effect on the photoresponsivity seems to be partially compensated by that in noise, leading to D* independent of L and W at wavelengths in the ultraviolet‐visible‐near infrared range. This result sheds light on the quantum physics underlying the optoelectronic process in QD/graphene nanohybrids, which is important to the design of high‐quality QD/graphene photodetectors and imaging systems. [ABSTRACT FROM AUTHOR]