Here is a little VBS function that will return the size of a folder in bytes

set oFS = WScript.CreateObject("Scripting.FileSystemObject")
 
'Example of Usage
wscript.echo GetFolderSize("c:\temp")
 
'The Function
Function GetFolderSize(folderpath)
   set oFolder = oFS.GetFolder(folderpath)
 
   GetFolderSize = oFolder.Size
End function