The body is the white canvas you see when designing a report.

So if you use A4 portrait, the height of the report is 29,7cm. If you have 1,5cm margins on each side, you have to subtract 3. So you have 26,7cm left for body.

If you have a header and/or footer, you have to subtract the heights from those as well. For example, we have 20cm left. The body can than have a maximum height of 20. If it becomes bigger, you might end up with extra pages when exported to PDf.

The same goes up for the width.