The most common approach to show Work Items Gantt is to create a single LiveReport page, call it for example Gantt, and put the Work Items Gantt Widget as single widget there.
In this case, we recommend to activate the following widget property:
Maximize- set the Maximize property of the widget to 'yes'
This will make the widget size equal to working area size.
Please make sure there are no additional empty paragraphs or other whitespace content around the widget.