Comment # 7 on bug 1209155 from Michael Hirmke

I also have a HiDPI notebook.
I have no real solution for the tty consoles yet, but as for the X11/KDE
environment I set

export GDK_SCALE=2
export GDK_DPI_SCALE=0.5
export XCURSOR_SIZE=48
export SPOTIFY_SCALE=--force-device-scale-factor=2
export QT_AUTO_SCREEN_SCALE_FACTOR=0
export QT_SCREEN_SCALE_FACTORS=eDP-1=2
export PLASMA_USE_QT_SCALING=1

in a KDE startup script in ~/.config/plasma-workspace/env/, where eDP-1 is the
internal display.

Except for XCURSOR_SIZE, which obviously doesn't have any effect, everything
has a usable size then.