You can find out the size of an individual folder by choosing the File > Get Info command from the Finder menu or by pressing the ⌘ + I keyboard shortcut. There is also a way to display the size of all your folders in the Finder window’s size column (by default, the Finder displays — in the Size column).
To do so, open a Finder window and choose View > Show View Options from the Finder menu or press the ⌘ + J keystroke combination. Click the checkbox next to Calculate All Sizes. This will caluclate the size of your folders and display it in the Size column.
If you wish to display the folder size in all your Finder windows, click the Use as Defaults button on the bottom of the View Options window.