Abstract: The plasma size has important influence on the power of the extreme ultraviolet source at the intermediate focus (IF point) and the design of the illumination system for a capillary discharge extreme ultraviolet source. The collection efficiency was calculated under different lengths of the plasma theoretically. Moreover, the spectra of the Xe plasma under different lengths of the plasma were obtained by a Rowland spectrometer. By combining the theoretical and experimental results with the design of the collectors in this system, the 13.5 nm (2% bandwidth) emissions at the IF point were calculated under different lengths of the plasma. The results show that the optimal power at the IF point and the optimal size of the plasma can be obtained when the length of the plasma is 3-6 mm.