Created attachment 3101 
xfce4-mixer with the window forced wide by the program

If one has multiple devices which xfce4-mixer can control, and one or more of the descriptions the system gives to those devices are long enough, the mixer program makes the drop down widget that lists them very wide in order to accommodate the longest items. 

This makes the window too wide in some cases, and prevents the user from reducing the window's width, even to the point of having more blank space in the window than space occupied by controls, if one doesn't have that many visible in any one tab.

Attached is a screenshot showing the issue.  The window's width is set as small as the program will let me, and the height is about where I'd normally set it.  There is even less space occupied by controls in the "Capture" and "Switches" tabs.

The user should, no matter what, be able to shrink the window width down to whatever size is desired, which would necessitate abbreviating the contents of the drop-down item as long as it collapsed, and perhaps horizontal scrolling if there are too many controls to fit.

There seems to be no problem adjusting the window's height.