I'm trying out Linux and different DEs now. KDE handles taskbar on the left side of the screen nicely, but I haven't found a way to make icons of running apps smaller so I can see their titles as well. All answers I found talk about quick launch icons only.

to change the size of the icons you need to change the width of the panel (click the cashew on the panel bottom and drag the "--width"). Iirc you won't achieve what you're looking for.

You can see there titles by hovering

No settings or plugins?

nope

if you want to have the names displayed you best move the panel to the top or bottom

ps moved to Plasma forum, maybe someone else has an idea