Obviously you have to co-ordinate the orientation with the paper feed of your printer and its printing margins (as in Page Setup).
You can have a wide sheet in printing that is portrait and a narrow sheet that is landscape, the terms simply mean the way the paper is turned in the Page Setup, that is whether it is going into the printer straight or sideways. Only you know how your printer is grabbing the paper.
Printing margins set up a non-printing area on your paper's edges. How much of the paper can be printed is a factor of your printer's mechanics, plus what you set as the margins in Page Setup… (not to be confused with the margins in your Document, which simply place the default text within certain bounds.