Quote:...I have specified should be the actual printed image size, regardless of the DPI resolution of the print itself? Maybe I'm simply mistaken in that assumption...am I misunderstanding something basic? ..
That is correct, you should get the required image size, unless the size has to shrink to fit the paper.
From your screenshots the preview looks about right, small non-print margins + two 2.843" + 7.0" = Paper width 13"
The image size is quite large, Gimp uses pixels so 7.0 x 4.692 @ 1303 ppi is 9123 x 6115 pix. Is that correct.
The 300 v 600 v 1200 is dots-per-inch, (economy mode <> quality mode) the rate at which the print head spits out ink droplets, not the same as pixels-per-inch which does govern print size. see: https://www.shutterstock.com/blog/dpi-vs-ppi
Looking at the CUPS setup. A4 page size Output resolution 600x600 dpi I think that is a quality setting. Print Area - Use Reduced Printing ????
Why there is a problem with the setup, I do not know. The ultimate solution is not use Gimp for printing. One solution is use, say LibreOffice.