There is a definite reason why Windows Explorer does not show the sizes of folders.
Because Windows Explorer is a file manager and finding sizes of folders is a time consuming operation. It will slow down the file management operations for which Explorer is designed. For the same reson, you should not even go looking for plugins for Explorer that find folder sizes.
You need special software like ShowSize that is designed for this purpose.
ShowSize goes through the complete hierarchy of folders and files and calculates the folder sizes to show to you in its Folder Sizes list.Click here to read more about the Folder Sizes list in ShowSize.