@JoEtzold; and one more thing to point out to you - my OP, I am not working with JPG, my output from Poser is PNG
We have only brought JPG into the topic as "Document Size" comparison as cameras output JPG or RAW in higher end camera.
I work with PNG or PSD format (sometime TIFF) to keep the higher resolution that is lost when compressing into JPG.
So again: Document Size (media size & Dpi) are the print quality of any given Pixel Dimension.
I bring this up as I was looking at your referred to links - they do not seem to address the Print Resolution of a given JPG.
For Document Size & Resolution it does not matter the Format of the Image: If I have a 180dpi 4x5 Document Property set on an image, I can save it as PNG, TIFF, PSD, or JPG and it will open in any other Image Editor as THAT size and and dimension. (Except for Poser, for Poser will calculate it back down to Dpi of 72 and what ever document size that correlates to the given Pixel Dimension.....hence the "oh bother")