Final Draft has two paper sizes, US Letter and A4. To switch from one to another, you can click the button on the right side of your screen:
From there, choose the “Layout” tab. There, you will see the “Paper Type” setting. You can choose between US Letter and A4 via the dropdown menu:
Scroll down for other layout options, like your margins. These can be adjusted using the “+” and “-“ symbols next to the “Top” and “Bottom” sections.