The A4 paper selected in the printer dialog has an aspect ratio of 210/297=0.707.
If your image is based on FF, it has an aspect ratio of 0.667, which is not equal to 0.707.
Increasing one margin at will while keeping the aspect ratio of the image intact will change the size of the image on the print as we can see. Other margin values don’t automatically change, which is pretty much the right thing to do if we think of these margins as being minimum values.
In order to solve the riddle, let’s make a little experiment: If you add equal margins on all sides step by step, you’ll come across a set of margins with a cell size aspect ratio of close to 0.667, your target value. See spreadsheet for illustration:
As we can see, the printer driver offers a cell size that is very close to the computed size - but only if I tell the driver to print borderless. If you now want to add some text below the image, reduce paper hight (spreadsheet line 5) accordingly.
Alternatively, you can set a cell size with an aspect ratio that corresponds to your image’s and see what you get. Note that there is ONE set of equal borders for each paper and image aspect ratio pair, if any (cannot print a square image on A4 with even margins)…
In the case of a 3:2 image printed on A4, the borders must be 18mm each.