PAH band ratios in the NW PDR of NGC 7023. The 6.2/11.3 ratio is usually used as a proxy for the ionization level of the PAH population, while the 3.3/11.3 ratio is expected to trace PAH size. Here we have plotted them for circumcoronene in blue, and circumcircumcoronene in red. The spectra for C₉₆H₂₄ derivatives are complete in this A_V range. The spectra for C₅₄H₁₈ species are missing only the small contributions from partially dehydrogenated species (read text). Observational studies have derived typical 3.3/11.3 ratios of 0.5 to 0.3 from the surface to the PDR up to distances of about 52 arcsec from the star. As seen in the figure, intermediate size PAHs could potentially reproduce these values. However, the ionization ratio 6.2/11.3 would then be highly underestimated compared to the observations that predict a value of 2 at the surface of the PDR, decreasing in about 14% to a distance of ~55 arcsec from the star.