Sort folders by size in Finder
If you open the folder in finder such that your sub-folders are displayed you can click on size at the top of the list. If the folder sizes are not displayed, open finder and under the View menu select "Show View Options". This brings up a pane like an image shown below.

  

At the bottom of the pane is a checkbox to determine folder size. Check the box and folder sizes will be displays. Then by clicking on the size near the top of the Finder window. the contents should be sorted by size. 

You need to make sure size is shown in the Finder window. To ensure this is the case, in the pane we just looked at, be sure to check "Size".