If you wish to always see a certain number of rows on reports, you can use data filters to set this. To do this, click on System preferences link within the System settings section of the My details menu. Reports that can be filtered in this way have a Data filter link at the base. Clicking on this link will take you straight back to the System preferences form to change the filters you wish to use.
If you just wish to set a different number of records for the report you are currently viewing, you can change the number at the top of the report and click the Search button to display the new number of rows. However, once you leave the report and come back, the old default number of records will then appear.
Please note: setting a large number of rows for a report can cause the system to timeout (large being more than 200).