Comment # 2 on bug 1001272 from Wolfgang Bauer

It's likely this: https://bugs.kde.org/show_bug.cgi?id=366299

The dialog is resizable though.
Of course, if the window is too big to fit on the screen, you may not reach the
corresponding border/edge to resize it.

You can still resize it by right-clicking on the window title bar and choosing
"More Actions"->"Resize. That's also available if you click on the icon in the
top left of the window or by right-clicking on the task bar entry.

Or you can move the window by clicking on any empty space inside the window (or
anywhere if you hold down the Alt key), hold it and drag.

Btw, I just measured it, and the dialog is "only" 670 pixels high here, should
actually fit on a 900 pixel high screen. How high is yours?