There are two ways to create mixed page sizes in InDesign CS5, but only one lets you change individual page orientations, too.
“How do I change the orientation of a new custom page size I add? I’m trying to make a tabloid-sized fold-out page. I have a facing-page letter-size document, and I used the dropdown menu in the Pages panel to add the tabloid page, but it’s portrait like the rest of the document. I can’t find the control where I can change the orientation of the new tabloid page to landscape.”