The answer to a question on https://gis.exchange.com I wrote that gave me the most fake internet karma*, talks about the location of the page properties in QGIS layouts. The question can be found at https://gis.stackexchange.com/questions/274148/setting-print-layout-to-portrait-using-qgis-3/274152#274152.
The page properties for layouts in QGIS, where you can change the page size and orientation in layouts are hard to find for users, especially in versions before QGIS 3.40. In that version the page properties were also added to the “Layout” menu, but in older versions it wasn’t available in the menubar.