I know of the 'trick' of "Pin - Alt+right mouse - Re-size - Unpin"; that has the desired effect until logout/login whereupon the Clipboard then has the default size again.

Is there any way to permanently resize the Clipboard? (klipper / plasma5-workspace v5.3.1)

Edit:
It seems from https://bugs.kde.org/show_bug.cgi?id=347210 that "... The fixed popup size is a conscious design decision ...".

Nonetheless, it is possible to resize, so, does anyone know how this setting can be saved.