IF the 5 pages are the only pages in the document--then go to 'document setup' to change the page size.
IF the 5 pages are only part of a multipage document (i.e. there are more than 5 pages) then select one page in the pages panel--then choose the custom page size icon at the bottom of the pages panel--click it--a dialog will pop up where you can choose or create a custom page size--then repeat for the other 4 pages