I have an HP 7612 wide format printer, and I would like to use the HP Photo Creations software to create and print a photo collage on 11 x 14 size paper. Unfortunately, 11 x 14 is not offered as one of the choices of layout size - the closest is 11 x 17. I tried using the 11 x 17 size and fitting all the photos to one end, but then part of the background is cut off.
Using the "Printer Settings" link on the Print panel in HP Photo Creations, I was able to specify a PostScript Custom Page Size of 11x14. However, the "Fit to Page" option under Advanced Settings didn't resize the layout to fit the invented paper size as expected. I'll check with our programmers.
Your technique of using an 11x17 layout is probably the best approach at the moment.
Another idea would be to
Set the image size to 5x7
Set the paper size to 11x17
Set Image Scaling to Fit to Page
That will print an 11x15.4 image, which may be closer to what you wanted.