The default margins for a report are from the margins set on the Settings » User Preferences » Reporting defaults tab.
To set margins for a report, complete the following steps:
On the Report Options dialog box, click the Layout tab.
Enter the margins in the Top, Bottom, Left, and Right margin fields.
Use decimals for fractions of an inch. (For example, 0.75 or 1.20).
After you set all margins, click Apply.