Please check following,
http://social.msdn.microsoft.com/Forums/en/sqlreportingservices/thread/0a6ab82f-adb0-462a-b649-867fa4915a92[
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Click Report Properties on the Report menu, switch to the Layout tab, and set the page size and margin after calculating. Please pay attention to the following rules:
1 Body + Right margin + Left margin should be less than page width.
2 Body + Top margin + Bottom margin should be less than page height.
3 If you have header and footer take them also into account.
4 If you have any textbox, line or other data regions, please make sure whether their width large then the size of page.