I’m using two 4K monitors, KDE desktop. After an update a long time ago (one year or more?) the standard Steam interface has minuscule text and corresponding small bitmaps. Many posts around suggest using env variable GTK_SCALE=2, mostly for Ubuntu, before starting Steam, but this doesn’t work either as a global variable or adding it before the steam command.
I was having the same problem with a new version of a CAD application, and finally bit the bullet and set the display global scale to 200% in System-settings. This has it’s woes, but worked for the CAD app. However the Steam interface is unchanged, with or without GTK_SCALE=2. Note that the configuration option in Steam to scale the interface according to the desktop never worked.
Today I stumbled on a workaround: If I change to big picture mode and then close its window, the standard interface comes back correctly sized! This is not permanent, at every restart Steam is minuscle again.
This at least indicates that a solution is possible, probably even easy, but I have no idea. Any suggestions?