Graphene-family materials exhibit excellent tribological behaviors as nanoadditives for eco-friendly water lubrication systems. However, the influence of lateral size, as a basic characteristic of graphene-family materials, on its tribological behaviors as nanoadditives is still unclear. Here, the tribological properties of graphene oxide (GO) nanoadditives with different lateral sizes are systemically investigated. The results show that different lateral sizes lead to different optimal concentrations of GO nanoadditives. Besides the mass ratio of GO nanoflakes, the amount of GO nanoflakes in lubricants is also critical for the tribological behaviors. Moreover, GO nanoflakes with smaller lateral size can more effectively reduce the wear of Si/SiO2 plates because smaller GO nanoflakes can more easily slip into the sliding interface to form tribofilms on the counterparts. This study provides fundamental insights into the size effect of graphene-family nanoadditives for water-based lubricants.