Here are the steps that has to be followed in-order to obtain Mac OS X Finder to show the size of your folders.
The first step require you to launch a new Finder window by selecting the Finder icon in your dock
Check the Calculate all sizes box.
Notice that Finder will now begin to calculate the size of your folders.
Note: This may take some time depending on the size of your folders.
You can now sort by size and actually see which folders are taking up the most space.