(right-click) any empty space on your desktop. Select Show View Options from the menu.
These shortcuts work in Finder but not on the Desktop. They also work in Open/Save dialogs if the dialog is set to icon view. how to make icons smaller on mac
In icon view, the View Options panel has a size slider; in column or list view, the slider affects icon size in the preview column only. (right-click) any empty space on your desktop