Folder.GetSize Method

public static long GetSize(DirectoryInfo sourceFolder)
 {
     long size = 0;
     FileInfo[] fis = sourceFolder.GetFiles();
     foreach (FileInfo fi in fis)
         size += fi.Length;
     DirectoryInfo[] dis = sourceFolder.GetDirectories();
     foreach (DirectoryInfo di in dis)
         size += GetSize(di);
     return size;
 }