I have noticed the fonts are way too small in Ubuntu 17.10 on my laptop with
2560x1440 screen size.
I found that people "solve" this by enabling fractional scaling in Gnome 3.26 on Ubuntu 17.10 "artful" in the Wayland session.
This allows you to pick the display scale out of a wider range:
100%, 125%, 150%, 175%, 200%. (by default there is just
$ gsettings set org.gnome.mutter experimental-features "['scale-monitor-framebuffer']"
Yet, by running
xrandr in Gnome Terminal I have noticed that the display resolution was just shrunk.
This was not desirable for me, so I revert the change:
$ gsettings set org.gnome.mutter experimental-features 
And increased a text scaling factor (default is
1.0), which allowed me to keep my resolution
2560x1440 while having a normal font size:
$ gsettings set org.gnome.desktop.interface text-scaling-factor 1.75
For some reason Gnome Text Editor (v3.22.1) did not pick the text scaling factor, so I had to increase the font size (from 13 up to 20) in its perferences menu explicitly.
Change console font size
With HiDPI display you may find your console font size is too small.
To change it:
$ sudo dpkg-reconfigure console-setup Encoding: UTF-8 Character set: . Combined - Latin; Slavic Cyrillic; Greek Font: Terminus Font size: 12x24 (framebuffer only)
setupcon in the console session to apply the new font size immediately or restart Ubuntu.
QT and HiDpi
When you are using QT and the fonts are too small, just export one of the following variables, adjust the numbers as you wish:
export QT_DEVICE_PIXEL_RATIO=2 (is deprecated since 5.4)
You can add one of these variables to
/etc/profile.d/qt-hidpi.sh file so they will get automatically exported for every logged-in user.