If you’ve noticed, Windows Explorer does not show the size of a folder in the Detail View, only the size of a file. Folder Size for Windows adds another column ‘Folder Size’ which displays the size of each folder in real time. Other columns that can be viewed and added are File Count which shows the number of files stored in each folder and Folder Count displays the number of folders within each folder.
To activate the columns, once you are in the ‘Detail View’, you need to right click on the column header and activate Folder Size like this:

(Screenshot is an example, I don’t have Folder Size installed here on my office desktop)