What is your suggestion for increasing the size of icons in the KDE system tray?
Next time, please start a new thread for separate issues, It makes things easier to find later.
Now, to answer your query.
Edit your ~/.config/plasma-org.kde.plasma.desktop-appletsrc file
After every line that commences with extraItems=, add a new line with iconSize=2
Then restart plasmashell:
kquitapp5 plasmashell && kstart5 plasmashell
The systemtray icons should now match the other icons on the panel, and have a much higher size limit when adjusting the panel height. If not, then just change the number in the iconSize= line, until your desired size limit is found.
Note: You may need to use Ctrl+c in the terminal if you get verbose output after the commands are run.
I have noted your advisory and explanation for changing the size of icons in KDE system tray.