why in LR is not possible to customize the size of the photo book pages? If acvquisto with Blurb stand corrected as I always have, using their formats, but if you want to make a PDF and then stamparmelo with my printer? How can I do if I want a book with dimensions 42 * 28 cm?
As you've discovered, the Book module does not allow custom page sizes or custom page templates. Over the years, it's been one of the more popular feature requests. Please add your vote and opinion to this feature request in the official Adobe feedback forum: Lightroom: Ability to create custom book templates/sizes from scratch | Photoshop Family Customer Community