An analysis has been carried out to determine the performance of porous radiant burners (PRB) as a function of fiber size. PRB made with silica or alumina fibers are considered. The radiative properties of the fibers are determined using the electromagnetic wave scattering theory for two different characteristics temperatures— 1000 and 1500°C. The properties are used in a combined-mode heat transfer model to calculate the amount of energy radiated by the PRB. It is found that fibers smaller than the order of 1 μm in diameter produce significantly higher radiant output. For a characteristic temperature of 1000°C, in some cases, the increase in output is as high as 63 and 109% for silica and alumina fibers, respectively. For a characteristic temperature of 1500°C, the corresponding increases are 72 and 150%, respectively.