To scale the font size of Spotify on Linux, open a terminal window and enter the following command.
spotify --force-device-scale-factor=1.5 <enter>
You will find that spotify menus are now bigger and more readable.
To make them scale permanently whenever Spotify starts, create a desktop shortcut and put the scaling factor in the desktop file.
sudo vi /usr/share/applications/spotify.desktop <enter>
This makes it scale the fonts in Spotify when Spotify is started.