Change Gantt chart width
Edit the page
...
Click the Gantt chart.
...
Change width.
...
Select the Page layout option on the Confluence editor toolbar
Choose your preferred layout and place the Gantt macro in the respective container
...
Change Gantt chart height
Go to edit mode of the Gantt chart
...
Open the configuration panel
...
Change view height value
...
Close the configuration panel by clicking the update button
...
Save the Gantt chart
...
...