Graphene oxide (GO) has been used as a particle surfactant because of their amphiphilic character. Recently, GO have been investigated in many reports for improving the dispersion stability of carbon nanotube (CNT) in polar solvent such as water or alcohols. As a surfactant material, GO should have dimension comparable with other nanomaterials interacting each other. Therefore, it is highly demanded to elucidate the size effects of the GO on the dispersion behavior of insoluble nanomaterials. However, the size effects of GO on the dispersion behavior of CNT dispersions or on the electrical and mechanical properties of CNT based composites have been rarely demonstrated. In this study, the size effects of GO on the dispersion behavior of MWCNTs, and the electrical and mechanical properties of PVA-MWCNT composites were investigated using GOs with different sizes. To determine the size effects of the GO, the size of GO was varied from micro-sized (2060 nm) to nano-sized (257 nm). By UV-vis measurements, it was found that smaller GOs improved the dispersion of MWCNT more effectively than larger one, and furthermore it was apparently demonstrated in the results of sheet resistance for MWCNT film that the minimum GO-to-MWCNT ratio to achieve a conductive GO-MWCNT film was gradually decreased as the size of GO reduced. Finally, the size effects of GO on the mechanical properties of PVA compo-sites were studied by measuring tensile strength and Young’s modulus, showing that the mechanical property enhancement effects were emerged more significantly from the smallest GO than the largest GO in this study. Our design of surface-engineered GOs combined with their significant advantages of 2D plate geometry in nanoscale and tunable optical and electrical properties highlight the importance of GO not only as a dispersing agent in producing functional colloidal suspensions, but also novel hybrid carbon nanomaterials for many applications, such as structural composites, transparent conductive electrodes, and energy storage elements.