A principally new, quantitative system of the classification of spectra of planetary nebulae is proposed. The excitation class of a nebula, p, is determined according to the relative intensities of emission lines, N1+N2/4686 HeII and N1+N2/Hβ. The excitation classes are obtained for 177 PN with known distances and sizes of all classes – low (p=1–3), middle (p=4–8) and high (p=9–12+). An empirical relationship between the excitation class p and the mean radius of the nebulae Rn is discovered – the largest sizes occur for highly excited nebulae, and the smallest for low excitation ones. This relationship as well as excitation class p, as an independent parameter admit an evolutionary interpretation (Gurzadyan, Egikyan 1991).