A general expression of the photoluminescence (PL)-spectral shape depending on the size distribution of quantum dots (QDs) in the condition of low doping concentration is presented, using the effective mass approximation and supposing a Gaussian distribution of QD sizes. Effect of the size fluctuation on the PL-broadened line is discussed. As an example, the PL-broadened lines of both IV-VI CdSe QDs and II-VI PbSe QDs are calculated. There is evidence to show that the calculated PL intensity, PL-peak wavelength and full width at half maximum of the PL-broadened line are in consistent with the experimental measurements. The size distribution of QDs has primary effect on the PL-broadened line. The PL-broadened line is an inhomogeneous broadening.