Page size change to 8.5 X 5.5

In order to upload a pdf file to an online printing company it must be the exact size of the book to be published and not "letter sized"  I need to make it 8.5 x 5.5 This is easy to do using CutePdf but can not discover how to do it using Adobe Acrobat. It will only allow me to designate the size of the printer paper but will not save the file in that size.