If you are planning to print your ARW report or export it to a pdf, the margins must be within the bounds of the paper. If you are exporting your report to a spreadsheet, you can expand the margins past this point.
To adjust the report margins double Settings option in the Report Fields panel.
On the Report Settings pop-up that appears, set the margins that you would like to use, then click the OK button to set the margins.
To expand the space available in the Report Layout section, click the right edge of the white report space then drag it to the right as far as necessary. Keep in mind that if you are printing the report, you should not expand past the width of your paper. If you are exporting the report to a spreadsheet there is no limit on the report width.