How do I set the width/height to fill/automatic?

I dont see any discussions about this.
I am using the drag and drop UI. How is there no fill option? How is there just px size?
If there is no fill implamented yet then the developers made a tool that has absolutely no usability.

There is no fill/automatic in the drag and drop UI.
IT is responsive so IT Will scale automaticly.

How will it scale automatically with a defined width/height unit?