It is not possible to resize widgets that open from panel, like for example Calendar, Tiled Quicklaunch, Menu.
It used to be possible with Alt+right mouse click but it doesn’t work now.
When calendar widget is on desktop I can see handles to resize:
but when calendar is on panel there are no such options:
Operating System: Manjaro Linux
KDE Plasma Version: 5.20.4
KDE Frameworks Version: 5.76.0
Qt Version: 5.15.2
Kernel Version: 5.4.80-2-MANJARO
OS Type: 64-bit
Same problem here (Debian Bullseye, KDE 5.20.4), it's really annoying...
I found a partial workaround for some widgets, open the Audio Volume, Notifications or something, pin it at top-right corner, press ALT+F3, More Actions|Configure Special Window Settings and add following properties:
Ignore requested geometry – Apply Initially – Yes
Maximum Size – Force – desired maximum size
Then resizing works, but the window opens at bottom center of screen, so move window to optimal position&size, open the configuration dialog again and add:
Position – Apply Initially – x×y
Size – Apply Initially – w×h
Unfortunately, it didn't solve problem with calendar (the window can be resized, but the calendar table is still same size) and the Application Menu is also affected by this settings (maybe you can find some Window Matching settings to exclude the Menu)
*** This bug has been marked as a duplicate of bug 332512 ***
*** Bug 433671 has been marked as a duplicate of this bug. ***