Epub to PDF: 5"x8" output

Hello folks,

I am trying to do an Epub to PDF, so that it is viable for print-on-demand. I just can't figure out how to specify the output as being 5"x8" with 1.5cm margins (480 x 768px content).

I'm confused with the input and output device selections, since the epub was not produced specifically for a given device (but does work precisely on Kindle paperwhite, my own test device). The output won't be for a device, per se, but be used as the PDF to create a print edition of the ebook.

Any thoughts on what settings to use to approximate the dimensions desired, are greatly appreciated.