Get size of scroll bars

Use GetSystemMetrics WinAPI function with SM_CYHSCROLL and SM_CXVSCROLL parameters.

procedure TForm1.Button1Click(Sender: TObject);
begin
  Label1.Caption:='Height of a horizontal scroll bar = '+
    IntToStr(GetSystemMetrics(SM_CYHSCROLL))+' pixels';
  Label2.Caption:='Width of a vertical scroll bar = '+
    IntToStr(GetSystemMetrics(SM_CXVSCROLL))+' pixels';
end;