Sorry, Thomas I just now noticed your post. Thank you for your opinion and advice.
However, the composition of the posters was mainly driven by aesthetics, and taxonomy had very little to do with how I decided to divide the species. With the amphibians it worked because salamanders and especially frogs have more compact shapes/silhouettes, so it still looks good if you put many of them next to each other. With the lizards it's a different story, i.e. they have long tail and I didn't want them to overlap (between species) because it doesn't look good and because it creates confusion as to which species is which. I also tried as much as I could to have a correct aspect ratio of the size of each species but I also wanted each drawing to be fairly big. I don't think many tiny drawings on a poster would look very nice. This meant that the smallest species still had to be big enough to see it well, resulting in the biggest species being quite big, hence reducing the space available for the number of drawings, if that makes sense? I also think that for most lacertids it is essential to have both male and female as they have quite well pronounced sexual dimorphism.
And last but not least, it was never my aim with this to cover absolutely all species but rather to make some nice posters showing some of the diversity of the European herpetofauna, e.g. there are no Natrix, Malpolon, Eryx, Xerotyphlops because they are neither colubrids nor vipers but they are also too few of them to be ebough for a poster. Also some species were never drawn for the guide to start with, or have only head or belly drawings, so you would have never had
all species on your wall anyway