Created attachment 173705 [details]
Screenshot of kcm_fontinst in System Settings showing what it looks like once the preview is finally at a reasonable size

SUMMARY
The font management page leaves very little space for the font preview, and there is no way to make it wide enough as well: the maximum size before System Settings shows both the top level and the category sidebars is still too narrow.

This was recently expressed on Reddit by u/PrathamChauhan_2612
 (https://www.reddit.com/r/kde/comments/1fbj7k8), but I have encountered the same frustration myself.

STEPS TO REPRODUCE
1. Open kcm_fontinst in System Settings
2. Try to make the preview wider (while keeping the font list wide enough to not ellipsize)
3. Note how it's not really possible, even when System Settings is maximized

OBSERVED RESULT
Font management in System Settings wastes tons of space on horizontal panels

EXPECTED RESULT
Font management, especially the preview

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: 
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION