@hajkan: PDF files can contain multiple page sizes, in general each page can have its own page size.
LectureNotes gives recommendations when you click at the `?´ aside `paper size´.
Like I said, it is hard to know the pdf's actual pixel size when importing especially on a mobile device where the number of pdf tools and their feature sets are limited.
For instance I was trying to create a new notebook from a weekly calendar template and man it was a struggle. I always had some space here and there. But if LN matched the fiorst page when creating the notebook size, my life would have been much easier at that time.