By using
$objPHPExcel->getActiveSheet()
->getPageSetup()->setPaperSize(PHPExcel_Worksheet_PageSetup::PAPERSIZE_A4);
we can set a sheet's page size, but how do you supply a custom Paper Size for this? I want to set it to 8.5' x 13.0', Letter is 8.5 x 11.0 while Legal is 8.5 x 14.0
Check the source code for the library you're using at https://github.com/PHPOffice/PHPExcel/blob/1.8/Classes/PHPExcel/Worksheet/PageSetup.php#L35-L100
Line of interest is this one in the comments.
* 14 = Folio paper (8.5 in. by 13 in.)
Which you can use:
$objPHPExcel->getActiveSheet()->
getPageSetup()->setPaperSize(PHPExcel_Worksheet_PageSetup::PAPERSIZE_FOLIO);
It doesn't seem like you can set custom sizes though, only the predefined ones.