Sometimes, smaller size is not always better, and looking for nanomaterials that offer better device performance requires consideration of their properties at the first stage. In this study, the effects of the size of polyethylenimine-capped AuNPs (PEI-AuNPs) and proteins on the immunosensor performances, namely, sensitivity and limit of detection, are examined. The size-effect investigation of PEI-AuNPs involves their modification on the surface of disposable screen-printed carbon electrodes to support primary antibodies and their ability to load secondary antibodies and redox probes to perform amplification in the immunosensor. The correlation of the average size, electrochemical activities, protein size, and device property of PEI-AuNPs is investigated. The synthesized PEI-AuNPs with different average diameters ranging from 4.7 to 44.9 nm are employed for the investigation. When the sensor surface forms a sandwich architecture, the detection employs the current response of Ag