On Xubuntu 16.04 with XFCE4 the GUI font is too large on a 144 dpi display and there is no way to change its size in preferences dialog, at least I can not find any way to set the gui font size to a smaller value. I remember there was some catastrophic scenario with qt5 and "high" dpi screens a few years ago, but I hoped that this was not a problem in 2017 - unfortunately still! I just want to cut some videos quick on Linux and tried to use Kdenlive, but was confronted with an unusable interface with absurdly large gui font - a very, very basic thing just simply does not work. Very disappointing.
You need systemsettings5 to change the font size and DPI for KDE applications, if XFCE does not provide a Qt plugin to do this automatically. You could also try the qt5ct tool, which is designed to offer such plugin for those minimal desktops. Please ask for help in a forum of your distribution.
I had the opposite problem, that fonts were too small, and are fine for me at 144 dpi. Nothing I tried would change the font size, including systemsettings5 or qt5ct. I ended up bodging it with this wrapper script to start kdenlive, to set the dpi to 144 then reset it afterwards: #!/bin/bash dpi=`xdpyinfo | grep resolution | awk '{print $2}' | cut -f 1 -d x` xrandr --dpi 144 (sleep 5 ; xrandr --dpi $dpi) & kdenlive $@