Changing page layout from portrait to landscape using ArcPy with ArcMap? is answered by saying that changing a layout size in ArcMap requires you to use .Net or comtypes rather than ArcPy (arcpy.mapping).
With ArcGIS Pro now available, can it be done there using ArcPy (arcpy.mp)?
This becomes easy in ArcGIS Pro where, since at least ArcGIS Pro 1.4.1 (and probably earlier), the Layout class of arcpy.mp has Read and Write Properties for pageHeight and pageWidth to set a layout's page height and based on the current pageUnits.
In ArcGIS Pro not only can you change Portrait to Landscape, but you can make your layout size be whatever size you want, from ArcPy using its arcpy.mp sub-module.