The XCURSOR_SIZE needs to be set to how many times you want to have it bigger times 16, so if your QT_SCALE_FACTOR is 2, set it to 32.
Now I’ve got 99%, Thank you so much tsujan!!!
The only thing that is missing is the size of the window-frames, especially the icons used to maximise/minimise windows. They are still small. I managed to do a workaround by doubling the font-size in the openbox-settings for the active window-title, inactive window-title and so on. At least this makes the window-titles readable, but the icons stay small.