Portrait, landscape & square pages
Page shape and key layout are separate choices. A landscape page can still have a tall, portrait key.
Watch tutorial video Opens YouTube · some videos show an earlier layoutPortrait
Gives you a tall page. This often suits a map of Great Britain or a north–south territory. After switching, check the map framing, title and key in Print View.
Landscape
Gives you a wide page. It can work well for an east–west region or a presentation slide. Move and resize the map and key to make good use of the width.
Square
Uses a square page for a more balanced local map or a square graphic. Check the edges before exporting; changing the page shape does not decide the best composition for you.