Note: These options ensure the image looks good at the reduced size and, to some degree, if you enlarge it, too. Tip: See below for setting up a keyboard shortcut for resizing.
Select Tools | Adjust Size… from the menu.Tip: To make sure the picture opens in Preview, click on the image with the right mouse button and select Open With | Preview from the context menu. To change the size of an image with the Preview application that comes with any Mac‘s macOS: