There is no simple single command to do that - but it isn't that difficult. The only complication is avoiding processing system files and folders when you iterate through folders (if you do, an exception will be thrown):
private bool IsIgnorable( string dir )
{
if (dir.EndsWith( ":System Volume Information" )) return true;
if (dir.Contains( ":$RECYCLE.BIN" )) return true;
return false;
}
Other than that, it's just a case for recursing through folders, getting the files and folders list, and processing them using the FileInfo class:
foreach (string s in Directory.GetFiles( dir ))
{
FileInfo fi = new FileInfo( s );
size += fi.Length;
}