I once made a drawing program for windows and also had this issue. There i used mm internally to make sure gfx remained same pixel size using GetDeviceCaps() with HORZSIZE and VERTSIZE, HORZRES and VERTRES. Im not sure how one would do the same in Qt / portable way.

Also as you mention. Even when working fairly ok, then the Use Large font in win7 + will often not be as expected. So maybe any workaround will come in handy on different systems.

If the user sets the size of the rectangle in pixels, then the font must be set in pixels, too. Otherwise you will have a "nicely framed text" on one display, and a "design disaster" on another system - based on exactly the same parameters.

The alternative would be to design both objects and font in "points". An approach I experienced with a different GUI framework - and troublesome to work with.

If you get right down to it, font is always device dependent. The question is: Which parameters are you observing?
In one case (pointSize), the physical size (millimeters) of the font is supposed to be the same across multiple devices. Meddle with the DPI (e.g. I use large fonts in Windows), and the deal is off. However, even when use use the system-suggested DPI, you will have different results on different systems when you measure the size in pixels.
In the other case (pixelSize), the size in pixels is guaranteed to be the same, but the physical size might vary.

There is no "perfect" solution. However, I hoped that at least, when I say 20 pixels, I get 20 pixels, no matter how big it actually looks to the user. Seems I was wrong...

As for your suggestions: Of course, I can find a workaround for this specific situation. I am rather trying to understand whether there is an underlying concept or issue I do not properly understand, or a known bug in that direction (not that I could find one).

*) When I write "extensively", I mean "over the course of several days.

So using setPixel is font device dependent and will change.

Update: I used Qt5.5
Maybe it was different with 4.8.7.
Also I did not check the values as it looked as expected.
It sounds like your case is more "tight" and hence its noticeable.

Its not an option to check heights with QFontMetrics and simply lower the size until it's within
acceptable Heights?

In general, the "point size" of a font is the size of the x-height + ascender area's size + descender area's size, and the point size says nothing and gives no restrictions about the width of each character. So, the only reason Arial is bigger than Times is because of its width. Any other size difference depends only on the visual style of the font design (in this case, the fact Arial has a bigger x-height).

I don't quite get the point you are trying to make. I have the same font on both systems, and I am only concerned with it's height. Plus, as said, I use pixelSize, not pointSize.

However, one thing occurred to me: Rounding might differ, because on Windows, qreal is defined as double, whereas on an embedded hardware without FPU, qreal is float.

Edit: Maybe I should elaborate: The differences on the OSes are the output of QFontMetrics::height() with the same font.