uses FileCtrl;

function DirSize(Dir: string): integer;
var
  SearchRec: TSearchRec;
  Separator: string;
  DirBytes: integer;
begin
  Result:=-1;
  if Copy(Dir,Length(Dir),1)='\' then
    Separator := ''
  else
    Separator := '\';
  if FindFirst(Dir+Separator+'*.*',faAnyFile,SearchRec) = 0 then
  begin
    if FileExists(Dir+Separator+SearchRec.name) then
      DirBytes := DirBytes + SearchRec.Size
    else
    if DirectoryExists(Dir+Separator+SearchRec.name) then
    begin
      if (SearchRec.name<>'.') and (SearchRec.name<>'..') then
        DirSize(Dir+Separator+SearchRec.name);
    end;
    while FindNext(SearchRec) = 0 do
    begin
      if FileExists(Dir+Separator+SearchRec.name) then
        DirBytes := DirBytes + SearchRec.Size
      else
      if DirectoryExists(Dir+Separator+SearchRec.name) then
      begin
        if (SearchRec.name<>'.') and (SearchRec.name<>'..') then
          DirSize(Dir+Separator+SearchRec.name);
      end;
    end;
  end;
  FindClose(SearchRec);
  Result:=DirBytes;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  DirBytes: integer;
begin
  DirBytes := DirSize('c:\windows');
  Form1.Label1.Caption := IntToStr(DirBytes);
end;