Different paperSize of QPrinter on Windows and Mac

[Lykurg]

why does printer.paperRect().width() return different values on Windows and Mac?

Specify your paper size! If not the default values of your system are used and there obviously your win ond osx have different settings. Look: QPrinter::paperSize()

Also the font is printed in different sizes.

Guess you are using point value to set the font:

Note: When rendering text on a QPrinter device, it is important to realize that the size of text, when specified in points, is independent of the resolution specified for the device itself. Therefore, it may be useful to specify the font size in pixels when combining text with graphics to ensure that their relative sizes are what you expect.