How-To: Show the size of folders in the Finder on Mac [Video] | 9to5Mac

Step 1: Open Finder
Step 2: In the menu bar select View → as List
Step 3: In the menu bar, select View → Show View Options
Step 4: Check the box next to Calculate all sizes

This is a Neat finder trick to show sizes for Folders in Finder. Saves a lot of time while cleaning up that extra low capacity SSD storage.