Conducting polymers are promising electrode materials for high-performance supercapacitors, however, their specific capacitance and stability need to be further improved for many important applications. The particle size plays an important role in determining the electrochemical performance of an active material, due to the size-relevant effects of nanomaterials, and exploration of the relation between size and capacitive property are therefore of key general importance. Herein, polypyrrole (PPy) nanoparticles with precisely-controllable particle size are synthesized by a chemical oxidation polymerization process, which enable us to systematically investigate the size-dependant electrochemical capacitance properties and analyze the underlying mechanisms. It is found that the highest specific capacitance (652 F g(-1)) was achieved from the mid-size sample with the moderate surface area, but not the smallest one with the highest surface area, as the charge carrier concentrations present remarkable effects on the capacitance performance of PPy nanomaterials when down to the critical size. The results presented here provide new insights on the rational design and optimization of the dimensions of polymer materials to greatly improve the electrochemical performance for advanced supercapacitor applications. (C) 2017 Elsevier Ltd. All rights reserved.