I am confused on how to use the maximum line length for word wrapping a label in QGIS 3. The dialog and documentation show an option to "Wrap lines to" and then allow selecting "Maximum line length" but where do you specify the length? 

I want to actually constrain the label to certain width (maybe even a rectangle) and have the lines split into multiple lines based upon the width.

Here is the documentation:

[![enter image description here][1]][1]

  [1]: https://i.stack.imgur.com/xc4l3.png