This is the problem once i faced long time ago and just wanted to share this knowledge, at the time of create new Jasper Report it asks us to take a report design, so we most commonly pick the page layout at the time of starting the report. it may be anything like picking portrait or landscape design.
Every thing is fine going but suddenly if the report page design may be little bigger or you may not have enough space to design. at this time situation you may want to change the Portrait page to Landscape or vice-versa. you may be thinking to take a new page layout and work on it, but that could be a bad practice.
So let me make it simple for you, in your iReport design or in your Eclipse report design we must, go to outline in your IReport, you can see <reportname>.jrxml. Right-Click go to properties. So now you can see the properties dialog of your report is opened, scroll down to the end. We can see the default size selection of your report. Click on edit properties change your preferred page size of your report.