Go inside the Workflow Guide and click on "Add Step"
Step popup width and height options are present in Image, Message and Inline step panels
By default the step popup width is 300 pixels. The minimum & maximum popup width is 250 and 1000 pixels and minimum & maximum popup height is 100 and 1000 pixels
This scroller comes on the step if the description exceeds the default popup height
This is how the step looks when popup width and height are at maximum