So far, it seems like all that is happening is that Print to File is making the print to file page the size of the printable are of the page size you declared; 7.147 x 6.897.

Then you say, 6 up. What does that refer to?  What size prints on the page? When you tick Don't change print size, that means don't change print size.
You say space between... what?
We spoke on the phone, and I explained the difference between page size and printable area...

Need information to proceed.   Page color sticks fine. print size is fine.
Fred

I don't know what this means?


I don't know what this means.

Max printable area is 7.147 x 6.897... so what adds to it?

Here's what I get

I set your page size to 7.375 x 7.125...  leaving me a printable area of 7.147 x 6.897

Now I add 6 vertical 2 x 3 prints, and add to them, 2 borders with a combined width of .09 inches, which when they surround the 2 x 3, the 2 x 3 becomes 2.18 x 3.18

I add the gray page color, and I print to file.  Works perfectly!!  See screen snap.

Here's where you lose me.
Automatically, Qimage spaces the images properly for evenness .
So automatically, by arithmetic, I get a left margin of .27 and a right margin of .27.
Add 3 images of 2.18 inches wide (6.54) and I get used width of 7.08, leaving unused space at .067 inches..  
I need space between image 2 and three, and another between image one and two.
My left over space to work with is 0.67 inches..... splitting that for the needed spaces between Image 1 & 2 and 2 & 3, the spaces are going to be 0.032 inches

Hope this helps
Fred