type
  TImageSize = record
  Width: Integer;
  Height: Integer;
  end;
 function ReadGIFSize(Stream: TStream): TImageSize;
 type
  TGifHeader = record
  Signature: array [0..5] of Char;
  Width, Height: Word;
  end;
 var
  Header: TGifHeader;
 begin
  FillChar(Header, SizeOf(TGifHeader), #0);
  Result.Width := -1;
  Result.Height := -1;
  with Stream do
  begin
  Seek(0, soFromBeginning);
  ReadBuffer(Header, SizeOf(TGifHeader));
  end;
  if (AnsiUpperCase(Header.Signature) = 'GIF89A') or
  (AnsiUpperCase(Header.Signature) = 'GIF87A') then
  begin
  Result.Width := Header.Width;
  Result.Height := Header.Height;
  end;
 end;
 procedure TForm1.Button1Click(Sender: TObject);
 const
  FileName = 'D:\test.gif';
 var
  fs: TFileStream;
  gifsize: TImageSize;
 begin
  fs := TFileStream.Create(FileName, fmOpenRead or fmShareDenyWrite);
  try
  gifsize := ReadGIFSize(fs);
  ShowMessage(Format('Breite %d Hцhe %d', [gifsize.Width, gifsize.Height]));
  finally
  fs.Free;
  end;
 end;