Can FrameMaker create multiple Master Pages with different pages sizes in a single FM file?
You need to break your content between files and use the Book function to apply numbering to make it look seamless. Sometimes, if you can, re-arranging the content so that any oversize pages come at the end of a chapter/file makes it easier to handle such a split of the files into the book.
Just to add an additional information here: While you cannot mix multiple page sizes (like A4 and A3), you can mix portrait and landscape. Just select the (master) page in question and go to > Format > Customize Layout > Rotate Page [Clockwise / Counterclockwise].