No, it is not. The page size is exactly what is specified on tab "PDF", option "Paper Size" if "Automatic Page Settings" is disabled. However, PDF export only downscales diagrams to fit into the desired size, it never upscales diagrams if there is excess space. And that is the way it is meant to be. Every half-way decent PDF viewer lets you scale your document to fit the chosen paper size on print, so simply stick with "Automatic Page Settings" enabled and let your PDF viewer do the scaling.