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;