We studied the effects that internal porous structures tailored using size-controlled copper nanosheets (Cu NSs) had on the electromagnetic interference (EMI) shielding performance of the resulting Cu NS films. Cu NSs were carefully synthesized as two-dimensional (2D) conductive fillers, and their size was controlled by adjusting the concentrations of the shape modifier (iodine), stabilizer (hexadecylamine) and reductant (glucose). Three kinds of Cu NSs had the size of 3.8 μm (Max. 10.0 μm), 8.9 μm (Max. 28.0 μm), and 12.2 μm (Max. 33.7 μm) and used as a conductive filler for shielding film by spray printing. The films had different surface coverages, film thicknesses, and EMI shielding effectiveness (SE) at a fixed loading weight of 0.6 mg/cm2. The smallest to largest Cu NSs exhibited EMI shielding performances of 6.3 dB, 43.6 dB, and 69.7 dB, respectively, so performance was directly proportional to size.