Dispersibility of graphene oxide (GO) as a nano-reinforcer plays a crucial role in polymer-based nanocomposites. However, it is still uncertain whether lateral dimensions of GO could affect their dispersibility in polymer matrix. In this work, crude GO (CGO) was fractionated into large sized GO (LGO), medium sized GO (MGO) and small sized GO (SGO), through in situ polymerization on the fractionated GO (f-GO) surfaces, caprolactam (CPL) was polymerized and polyamide 6 (PA6) grafted GO (g-GO) was prepared, and then PA6/g-GO nanocomposite fibers were prepared through melting spinning. It is obvious that the dispersibility of GO, whatever f-GO or g-GO, increases with the decrease of its size. Particularly, SGO and PA6 grafted SGO (g-SGO) disperse much more uniformly than CGO and the other f-GO and their corresponding g-GO. As a result, PA6/g-SGO nanocomposite (SPA) fiber has outstanding mechanical properties, for example, the strength of the fiber is 5 times more than that of pure PA6 fiber.