How to set relative width of Panels within a row in Grafana 5 or 6?

In the documentation I found the following paragraph:

Rows are always 12 “units” wide. These units are automatically scaled dependent on the horizontal resolution of your browser. You can control the relative width of Panels within a row by setting their own width.

http://docs.grafana.org/guides/basic_concepts/

Where can I find this option in Grafana 5 or 6?
Thank you.