Updated June 5th, 2019.
If you have a photo or other image file and want to print it so it fills a page, open it with Preview and look for the “Scale to Fit” option in the Print… dialog box. If the file opens with some other app when you double-click it close it, then Control-click on the file’s icon and choose “Open With” and then “Preview” from the contextual menu.
This is by far the simplest way to print an image that fills the page. No other app makes it so easy.
The grayed-out percentage (215% in this screenshot) shows you how much the image was reduced or enlarged in order to fill the page. FYI.