Taken from here.
In the .config/lxsession/Lubuntu/desktop.conf change the following line:
iGtk/CursorThemeSize=72Lubuntu: How to change font size in QT applications
Fonts in QT apps in Lubuntu on high-res displays are too small.
Solution is to add these 2 lines at the end of .basrc:
export QT_QPA_PLATFORMTHEME="qt5ct"
unset QT_STYLE_OVERRIDE
Then install qt5ct:
$ sudo apt-get install qt5ctThen start qt5ct and configure font size to match your display resolution.