There are two ways of doing this, one using Windows Explorer and one from the command line. Using Windows Explorer:
Start Windows Explorer (Win key + E or Start – Programs – Explorer)
Right click on the respective folder and select Properties
Under the General tab a size will be displayed and this is the total size of the folder and all sub-folders and their contents.
From the command line you can just use the dir command with /s qualifier which also lists all sub-directories, e.g.
would list all files/folders in the d:\vhosts\users directory and at the end the total size.
NOTE: This will report the total of the file sizes, but not the amount of disk space. If for example any of the files are compressed, then the disk space consumed will be less that the size reported.