After updating to Fedora 26 from 25, the window title-bars in KDE Plasma 5 were scaled up way beyond what they should be for me.

Took a minute to find out what to do since the option to resize these button is very hidden away. Theme-dependant too!

The fix:

Navigate: System Settings -> Application Style -> Window Decorations Click one of the themes context menu icon
Navigate: Window Specific Overrides -> Add
Click "Hide window title bar"
Use .
for the regular expression to match all windows*

The original link is:
https://www.reddit.com/r/unixporn/comments/4gez8w/hidetitlebarsinkdeplasma5/