Get Summary Info on Multiple Files

If you select multiple files in the Finder, using either the Shift or Command key as you click the files, you can get a special Multiple Item Info box to appear. After you have them selected, Go to File, Get Info but before you select it hold down the Control key. The menu item changes to Get Summary Info and the result is a single info box that combines data, like the total file size. The keyboard shortcut is Control+Command+I. If the files use the same extension, you can also see and change the Open With information for all of them at once.