Open application-launcher menu (click on kde-icon in taskbar)
When the menu is open, resize the menu using alt+right mouse and drag the panel to the desired size.
Similarly, open then status & notification panel, click on the 'pin' icon, then resize the popup with alt+right mouse button.
(this need not to be performed in combination with the app-launcher, the error also occurs if only one of them is resized)
As long as you do not log out, the panels keep their (resized) sizes.
Now, log out and re-log in again.
Both panels will be automatically resized to their default sizes, any resizing information of the last session is lost
*** This bug has been marked as a duplicate of bug 332512 ***