Aims: In this paper we study the influence of the ionizing cluster mass on the emission line spectrum of H ii regions in order to determine the influence of low mass clusters on the integrated emission line spectra of galaxies. Methods: For this purpose, we present a grid of photoionization models that covers metallicities from Z = 0.001 to Z = 0.040, ages from 0.1 to 10 Ma (with a time step of 0.1 Ma), and cluster initial masses from 1 to 107 M_&sun;. The stellar masses follow a Salpeter initial mass function (IMF) in an instantaneous burst mode of star formation. We obtain power-law scale-relations between emission-line luminosities and ionizing cluster masses from the grids and we evaluate the dependences on the ionizing cluster mass for some line luminosities, equivalent widths and line ratios. Results: Power-law scale-relations are shown to be useful tools to obtain robust diagnostics, as examples: (a) Hα/Hβ ratio varies from the usually assumed value of 2.86, these variations imply the existence of a lower limit to the attainable precision in extinction estimations of ΔE(B-V) ~ 0.1; (b) EW(Hβ) is a good age indicator with a small dependence on cluster mass, while EW([O iii] 5007) shows a noteworthy mass dependence; (c) abundance estimations from R23 are practically unaffected by variations of the cluster mass; (d) estimations from S23 and η' would improve if the cluster mass dependences were considered; and (e) [O ii] 3727/Hα is a good star formation rate indicator for ages older than ~4.5 Ma. We also show that the ionizing cluster mass dependence explains why empirical calibrations produce more reliable diagnostics of some emission lines than photoionization models grids. Finally, we show preliminary results about the contribution of low mass clusters (M < 104 M_&sun;) to the integrated emission line spectra of galaxies, which can be as high as 80% for some relevant lines.