If your desktop is unlocked (you can do that by Alt+D, L), a small Plasma logo will appear in the bottom right corner in the panel (it is commonly named the “cashew”). Click on this cashew, and the panel controller opens.clicking on the desktop, or when no application has the focus, with
The panel controller allows you to reposition, resize and realign the panel. The Plasma widgets living in this panel will adjust their size automatically. Plasma widgets have basic knowledge about sizing, provided by the containment. They are programmed to take advantage of that size, and inform the applet about how much space they possibly need. In the end, the containment gives a possible size to the applets, the applets obey.