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tokens: from the Coronene, Ovalene, Anthracene, Tetracene families ( Ricca et al. 2012 ; Peeters et al., in prep.), and the Pyrene family. While most of these PAHs are very compact, we have also included a few non-compact ones, to investigate the effect of the molecular structure on the 3.3/11.2 m ratio. All the PAHs in the selections are shown in Fig. A.2 ). As the UV absorption cross-sections for all PAHs are very similar, the uncertainty produces a vertical shift in Fig. 5 . Figure 5 shows that the intensity ratio is sensitive to the size of the PAHs ( Allamandola et al. 1985 ) and forms a well-defined relationship ( Ricca et al. 2012 ). The slope and shape of the relation is not sensitive to the stellar input spectrum or the extinction in the PDR. However, it can be shifted vertically for different average photon energies or adopted intrinsic strength. 4.2. PAH size With the model (Fig. 5 ), the 11.2/3.3 m ratio can be converted to PAH sizes. The horizontal colored lines in the figure have the same color coding as the 11.2/3.3 m intensity ratio in the maps. In the NW PDR (orange in Fig. 2 ), PAHs have a typical size of 50 carbon atoms. Moving towards the star, the ratio drops (green) and PAHs have average sizes of . In the south PDR the average sizes of the PAHs are slightly larger with respect to the NW PDR, (green) in the PDR surface, and increase towards the left up to (light blue) in Fig. 4 . The results show that the 11.2/3.3 m ratio is mainly sensitive to size and has little dependence on molecular structure. Matrix isolation and gas phase experiments, as well as theoretical density functional theory (DFT) calculations show that the intrinsic band-strength ratio of the C-H stretch to the out-of-plane-bending mode (3.3/11.2 m) is very constant between different PAHs ( Joblin et al. 1994 ; Langhoff 1996 ). However, the ratios derived by these different techniques are different. Further experiments are needed to settle this issue. However, we emphasize that a different adopted band-strength ratio will only produce a vertical shift in Fig. 5 . An uncertainty in the adopted intrinsic strength translates to a systematic uncertainty in quantifying PAH size. If the adopted intrinsic strength is increased by a factor of 2, then a range of –105 would change to a range of –80. The trends, however, remain. Fig. 5 11.2/3.3 m intensity ratio of 27 different PAHs with blue coronene family, green compact, and red non-compact PAHs. The average photon energy absorbed by the PAHs used is 6.5 eV (see Eq. ( 1 )). The ratio of the C-H stretching mode (3.3 m) over the solo out-of-plane bending mode (11.2 m) is extracted from the theoretical spectra from the NASA Ames PAH database. The -axis is linearly proportional to the adopted intrinsic band strength of the 11.2/3.3 m ratio. The colored horizontal lines have the same color coding as used for the 11.2/3.3 m ratio maps in Figs. 2 and 4 . 4.3. PAH size variation in the PDR Figure 6 shows a crosscut across