Be aware that you can't just tell a PDF to resize its page - you have to rebuild the entire PDF with new images. So I can certainly support what you are asking for, but the PDF will be much larger than the original (due to the lack of support for compressed images in the PDF library that I use), it will load slower, and the export will take much longer. Also, instead of letting the user enter a number for the page width, I'd rather just force every page to either be as wide as the page with the largest width, or as wide as the page with the smallest width. That's much simpler. I imagine most users would prefer that pages are enlarged rather than shrunk.
Mike
Mike