To change the page format of your score, click the main tab "Score". Select the "Margins" option from the "Layout" group.
A layout dialog will open, in which you can adjust the page format.
If you change the page format, then you need to revise all layout settings and if necessary margins, to adjust the score to the new page format. After making your changes, click "apply"
You can also adjust the spacings of your score by clicking on the "Spacing" tab.