This study investigates the combustion of single iron particles under low-oxygen (10–17 vol%) dilution conditions, focusing on the development of a novel in situ particle sizing method. The method measures the characteristic timescales of micron-sized iron particle combustion by probing the light emission intensity during melting, which is proportional to the particle's cross-sectional area. The study validates the experimental method through calibration and validation processes, demonstrating its effectiveness in measuring the combustion time of both spherical and irregular iron particles. The experimental results show that the total time of light emission (ttot) and the time to the maximum light emission intensity (tmax) follow power-law correlations with particle diameter, with exponents in the range of 1.47 to 1.87 and 1.48 to 1.72, respectively. A theoretical model is derived to describe the diffusion-limited burn time of growing (iron) particles, suggesting that the burn time scales with the square of the initial particle diameter. This generalized d²-law accounts for particle growth and predicts a significantly shorter combustion duration compared to the conventional d²-law for shrinking particles. The theoretical model is validated by comparing the predicted burn times with experimental data, showing good agreement. This validation confirms that the combustion regime of micron-sized iron particles under low-oxygen dilution conditions is limited by external oxygen diffusion during the intensive burning stage. The study highlights the importance of considering particle growth in combustion models, providing valuable insights for the design of future iron-fueled combustors.This study investigates the combustion of single iron particles under low-oxygen (10–17 vol%) dilution conditions, focusing on the development of a novel in situ particle sizing method. The method measures the characteristic timescales of micron-sized iron particle combustion by probing the light emission intensity during melting, which is proportional to the particle's cross-sectional area. The study validates the experimental method through calibration and validation processes, demonstrating its effectiveness in measuring the combustion time of both spherical and irregular iron particles. The experimental results show that the total time of light emission (ttot) and the time to the maximum light emission intensity (tmax) follow power-law correlations with particle diameter, with exponents in the range of 1.47 to 1.87 and 1.48 to 1.72, respectively. A theoretical model is derived to describe the diffusion-limited burn time of growing (iron) particles, suggesting that the burn time scales with the square of the initial particle diameter. This generalized d²-law accounts for particle growth and predicts a significantly shorter combustion duration compared to the conventional d²-law for shrinking particles. The theoretical model is validated by comparing the predicted burn times with experimental data, showing good agreement. This validation confirms that the combustion regime of micron-sized iron particles under low-oxygen dilution conditions is limited by external oxygen diffusion during the intensive burning stage. The study highlights the importance of considering particle growth in combustion models, providing valuable insights for the design of future iron-fueled combustors.