If I put a pager with virtual desktops in the main panel (taskbar), regardless of how I resize it when putting it outside the taskbar panel, it returns to the same size when moving it to a panel. Would be nice it there was some functionality for resizing this widget, which tends to take a lot of space. Also would be nice for it to group desktops in rows and columns to take less space.
You can configure the number of rows in the virtual desktop settings in System Settings. Widget resizing would be a matter of the panel.
Similar/duplicate issue https://bugs.kde.org/show_bug.cgi?id=348792
I think a functionality to resize widgets in the panel is, right now, an overkill. It's more appropriate to use margin areas as recently introduced.