Size and position cannot be set by expressions or custom code and even so you'd have to have chosen portrait or landscape at design time so the report will still print in that mode.

There isn't actually any property for portrait or landscape, it is determined by the PageWidth and PageHeight properties, the dialog just adjusts these. These are set at the report level and are also not able to be set by expressions.

Most importantly SSRS does not know the number of pages the report has until it's rendered them so you can't then tell it to go back and change something. It's why using total page numbers in the header or footer prevents SSRS from rendering the first page until the report is fully rendered.