How do I change the size of a photo for printing on a Mac

Print the photos directly from Photos with ⌘P :

Then you can size the printed photo by specifying the size you want on the paper, for example:

Set the layout to "custom" and then specify the printed width and height on the paper:

You have a photo selected, when you try ⌘P? There should be a "custom" layout somewhere among the templates. The pane with the templates can be scrolled. If there is no scrollbar to the right, try to scroll the pane anyway.

Post a screenshot of what you do see after using ⌘+P.