"Letter" or "A4" page size option grayed out in the "Set Page Boxes" menu

Employee combined PDFs into one document for a client which led to some pages being double standard letter or A4 sizing. When doing the crop + enter trick to set the page boxes, the option to resize the page to be Letter or A4 size is grayed out. How can I renable those page size options or set all pages to be the correct sizing.