We present a theoretical study of the optical gain, emission wavelength, and threshold current density for edge emitting laser structures with an active region based on InGaAs/InGaAsP quantum dots. The analysis of the effects of the size distribution of the dots is also presented. Spectral gain curves are generated for InGaAs/InGaAsP dots where high optical gain and high independence of spectral characteristics are obtained for a uniform distribution of dots. With typical non-uniform distribution, we show a reduction in gain by a factor of 6. Also, we predict the onset of new transition peaks and a red shift in the most probable operating lasing wavelength. Finally, we demonstrate that there is a large range of full width at half maximum (FWHM) of the dots size distribution where variations in the maximum gain and associated wavelength, as well as threshold current density, are minimum.