We report the dynamic crossover behavior in metallic glass nanoparticles (MGNs) with the size reduction based on the extensive molecular dynamics (MD) simulations combined with the activation-relaxation technique (ART). The fragile-to-strong transition of dynamics can be achieved by just modulating the characteristic size of MGNs. It can be attributed to the abnormal fast surface dynamics enhanced by the surface curvature. By determining the potential energy surface, we reveal the hierarchy-to-flat transition of potential energy landscape (PEL) in MGNs, and demonstrate the intrinsic flat potential landscape feature of the MGN with size smaller than a critical size. Our results provide an important piece of the puzzle about the size-modulated potential energy landscape and shed some lights on the unique properties of MGs in nanoscale.