Increasing mechanical flexibility without sacrificing electrochemical performance of the electrode material is highly desired in the design of flexible electrochemical energy storage devices. In metal-related materials science, decreasing the grain size introduces more grain boundaries; this stops dislocations and crack propagation under deformation, and results in increased strength and toughness. However, such a size refinement effect has not been considered in the mechanical properties, particle stacking, wetting, and electrochemical performances of flexible supercapacitor electrodes. In this paper, MXene was used as an electrode material to study the size refinement effect of flexible supercapacitors. Size refinement improved the strength and toughness of the MXene electrodes, and this resulted in increased flexibility. Finite elemental analysis provided a theoretical understanding of size refinement-increased flexibility. Moreover, the size refinement also improved the specific surface area, electric conductance, ion transportation, and water wetting properties of the electrode, and the size refinement provided highly increased energy density and power density of the MXene supercapacitors. A highly flexible, water-proof supercapacitor was fabricated using size-refined MXene. The current study provides a new viewpoint for designing tough and flexible energy storage electrodes. The size refinement effect may also be applicable for metal ion batteries and electronic and photo devices composed of MXene and other nanoparticles. (C) 2021 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.