I have a tiny Bash script to patch the icon size in the System Tray one size up (from 1 to 2). I run this script every time an update reverses my patch.
It patches the value of the <default> tag within XML tag <entry name="iconSize" type="Int"> in file "/usr/share/plasma/plasmoids/org.kde.plasma.private.systemtray/contents/config/main.xml". Then I restart Plasma and KWin, or log out and back in.
Quite a while ago this stopped working. And the xml file appeared to have been minimised, because most of the interesting settings (including System Tray icon size) were removed.
I grep'ed folder "/usr/share" for "iconSize" with "grep -rnwI /usr/share --colour --include=*.xml -ie 'iconSize'", and found it in file "/usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/config/main.xml". But that icon size is described as being for "The icon size to use for Folder View icons.". I decided not to chase this further.

But it still bugs me, because I find the icons too tiny. So where has this setting moved to ?