I've seen a lot of posts about how to convert a multi-page PDF to PNG but I'm wondering how I can go in the opposite direction if I have the size of each page I want made.
Print the PNG files to PDF. By default, the PDF pages will all be the same size but you can crop them down once it is created.
Assuming you have PNG files
page9.png and you want to create a 9-page PDF from these input PNGs.
The following simple ImageMagick command does create this PDF:
convert page*.png 9page.pdf
If you want to re-sort the pages, just list all the PNGs in the order you want:
convert \ page8.png \ page7.png \ page6.png \ page9.png \ page1.png \ page2.png \ page3.png \ page4.png \ page5.png \ 9page.pdf