How do I increase the report area?

When I use the Design View to create a report, then expand the Detail portion of the report, a horizontal bar is shown that appears to limit the size of the useful area when designing the report. Increasing the size of the Design area doesn’t affect the useful area. I have experimented and researched how to increase the useful area, but come up empty handed. Does anyone know how to adjust the useful area in a report? I have attached a screen shot of the useful report area I would like to increase:

Save the (empty) report, close the report and open it again.

Seems there is a default max value, which doesn’t recognize the real height of the page.
Have tested here: Detail section could be increased up to 26,41 cm here. Margins on top and button are set to 2 cm. But the page is only 29,7 cm. So the detail section could be (erroneously) set to more than one page…