• Quantum confinement effect critically governs charge transport, exciton dynamics, and emission efficiency in CsPbI3 QD-LEDs.
• Smaller QDs exhibit enhanced efficiency, while larger QDs provide increased brightness and stability under high current densities.
• Size-dependent trade-offs in LED performance are systematically revealed, offering design guidelines for optimal QD sizes.
• Findings pave the way for scalable and energy-efficient optoelectronic devices based on quantum dot technology.