As promising electrode materials for supercapacitors, manganese oxides still have big challenges such as the low material utilization and poor ionic/electronic conductivity. Reducing particle sizes through nanotechnology has been used to improve material conductivity and electrochemical active sites at materials/electrolyte interfaces. Nevertheless, the extremely small particle size may result in physical and/or chemical instability, mass loss and subsequent capacitance attenuation. Understanding this tradeoff effect of electrode materials size with their electrochemical properties is critical to fabricate highperformance supercapacitors. In this work, we prepare homogenous and size-tunable MnO particles (with mean diameters of 80, 41, 20, 15 and 9 nm) on carbon cloth via a facile gel-like film assisted method. It is found that the medium-size nanoparticle (20 nm) displays the best performance instead of the smallest one. These observations are different from the traditional view about material size-property relationship. Instead this work provides a new insight referring to both the size-dependent solubility and ionic/electronic transport. Beneficial from the good flexibility and high conductivity/stability of carbon cloth, the optimized MnO/carbon cloth electrode demonstrates extraordinary performance in symmetric solid-state supercapacitors with energy densities of 86 and 70 Wh kg(-1) at the power densities of 450 W kg(-1) and 9 kW kg(-1), respectively.