The only dimensions D|S understands right now is pixels. Are you wanting to save in "print quality" size? If that's it, then find out what resolution you want to print at (150 dots per inch, 200 dpi, 300 dpi, whatever) and simply multiply to find out the pixel dimensions you need to plug into the Render Options dialog. When you print the rendered pic, there should be a box to enter the resolution: enter the dpi figure you started with (150, 200, 300 etc) and the print size should snap to the page size you want.
I use an old version of Paint Shop Pro for printing, your program might be different.