By default Finder does not list the sizes of all items on your hard drive when viewing folders and files. If you’re curious about the size of an item and Finder simply places an -- in the Size column give this a try.

Bring up the View Options for the current folder you’re looking at, do this by either using Cmd+J, View > Show View Options or by right-clicking in a blank part of the Finder window and choosing Show View Options from the menu. Once there you’ll see a window like the one below, check the Calculate all sizes checkbox and close the window.

For an additional bonus, click the Use as Defaults button to apply this change to all subfolders. For one reason or another Finder does not remember this setting after logging out and back in, I would assume thats to minimize the work the CPU and hard drives need to do while browsing your files to make the OS snappier. If you only care about the size of a single file or folder, Quick Look will give this info to you with less effort, additionally the Get Info option under the File menu (also available by right-clicking the file/folder) will tell you a little more.