The controls for configuring wallpaper remain the same size even when the window is maximized. They have no ability to scale to accommodate larger font-sizes set by the KDE user. The main issue is how larger fonts appear within drop-down-menus, as shown here:
Notice all the wasted available horizontal space that the controls are incapable of utilizing (emphasized with red arrows). Notice that items in the drop-down menu are truncated and elipsified.
Elipsification should be a last resort when dealing with larger font-sizes. Why must the drop-down menu's selections be the exact same size as the drop-down menu? They should be as wide as necessary to avoid truncation instead. When the drop-down is selected, those items should not be truncated and elipsified. There is plenty of space available to showcase those phrases without any truncation. To often, with these inflexible drop-down controls, too much gets truncated; so much that you can even guess what the completion of the phrase is.
I don't know where you're getting these drop-down-menu controls, but they're very inflexible.
To reproduce, simply increase the desktop's font sizes to larger than 10pt (which is a very small default in my opinion). At the very least, the developers should choose larger font-sizes (than the default) while they're working on these controls. So, they can become aware of how poorly these drop-down-controls deal with larger font-sizes set by the KDE user. The larger the font used, the more ridiculous these unnecessary truncation become.
Look at the drop-down-controls that appear on websites, they never truncate long phrases; the drop-down becomes as wide as is necessary to showcase each selection without truncation. That's the way these drop-downs should work too. Also, truncation is unavoidable, hovering you mouse over that selection should produce a tool tip that displays that selection in its non-truncated entirety.
*** This bug has been marked as a duplicate of bug 403153 ***