For the KDE Plasma 5 desktop itself, this archlinux page might be helpful to you. Essentially scaling the desktop and/or effective DPI to suit. (I would not recommend changing the display resolution for any thing other than the native resolution.)
it looks like the root (default) resolution/dpi is different then the user chooses
happened to me on my old monitor I’d say if an nvidia card run nvidia-settings as root and save your resolution/dpi or set your resolution manually as deano_ferrari said or in /etc/X11/xorg.conf (depreciated but still works)
if it happens on the lock screen too it’s definitely sddm related just keep in mind if editing /etc/sddm.conf
an update will most likely remove your edit, I edited it to enable Num Lock but an update removed it