You undoubtedly know that you can figure out how much disk space a file or folder takes up by selecting it and choosing File > Get Info. But what if you have a bunch of files and/or folders whose sizes you want to add up, perhaps to see if they’ll fit on a USB drive or can be attached to an email message? Select all these items at once (Command-click them, for instance), hold down the Option key, and choose File > Show Inspector (or press Command-Option-I). Instead of a Get Info window, you see a floating Multiple Item Info Inspector that tells you how large the collection is. Better yet, if you add or remove items from the selection by Command-clicking, or make an entirely different selection, the Inspector adjusts to the new selection.