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The present work demonstrates the efficient design of ultrasmall porous carbon nanospheres with tailored sizes (5-40 nm in diameter) and optimized intrasphere textural properties for high-rate high-energy supercapacitor application. The carbon nanospheres are synthesized via a miniemulsion polymerization technique followed by KOH activation. It is shown that dual-step activation renders enlarged intrasphere micropores/mesopores, facilitating enhanced ion transports. Meanwhile, a decrease in nanosphere size from 40 to 5 nm significantly improves the rate performance, demonstrating the pronounced size effects due to enhanced intrasphere ion transport. The optimum dual-step-activated carbon nanosphere sample with an average sphere size of 5 nm, ACNS5-2, shows the high specific capacitances along with outstanding high-rate capabilities in both aqueous (272 F g^-1 at 0.5 A g^-1 and 81.6% of retention at 200 A g^-1) and EMIMBF₄ (223 F g^-1 at 0.5 A g^-1 and 67.2% of retention at 100 A g^-1) electrolytes in symmetrical two-electrode cells. In EMIMBF₄, ACNS5-2 displays a high energy density of 48 Wh kg^-1 at a high power density of 14 kW kg^-1, suggesting excellent energy storage efficiency. Moreover, the performance of ACNS5-2 competes well with or is superior to some best-performing porous carbon-based materials reported in the literature for supercapacitor applications even at lowered temperatures (at -20 °C: 150 F g^-1 at 0.5 A g^-1 with a capacitance retention of 64% at 10 A g^-1) and high mass loading (8 mg cm^-2: 205 F g^-1 at 0.5 A g^-1 with a capacitance retention of 64.5% at 20 A g^-1). Our results, combined with structure-performance relationships, offer valuable guidelines for the rational design of carbon nanomaterials of optimum supercapacitive performances.