Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

function Get_File_Size1(sFileToExamine: string; bInKBytes: Boolean): string;
 { 
 for some reason both methods of finding file size return 
 a filesize that is slightly larger than what Windows File 
 Explorer reports 
}
 var
   FileHandle: THandle;
   FileSize: LongWord;
   d1: Double;
   i1: Int64;
 begin
   //a- Get file size 
  FileHandle := CreateFile(PChar(sFileToExamine),
     GENERIC_READ,
     0, {exclusive}
     nil, {security}
     OPEN_EXISTING,
     FILE_ATTRIBUTE_NORMAL,
     0);
   FileSize   := GetFileSize(FileHandle, nil);
   Result     := IntToStr(FileSize);
   CloseHandle(FileHandle);
   //a- optionally report back in Kbytes 
  if bInKbytes = True then
   begin
     if Length(Result) > 3 then
     begin
       Insert('.', Result, Length(Result) - 2);
       d1     := StrToFloat(Result);
       Result := IntToStr(round(d1)) + 'KB';
     end
     else
       Result := '1KB';
   end;
 end;

 {****************************************************************************** 
Thanks to Advanced Delphi Systems here's another method which works just as 
well returning the same results 
*******************************************************************************}
 function Get_File_Size2(sFileToExamine: string; bInKBytes: Boolean): string;
 var
   SearchRec: TSearchRec;
   sgPath: string;
   inRetval, I1: Integer;
 begin
   sgPath := ExpandFileName(sFileToExamine);
   try
     inRetval := FindFirst(ExpandFileName(sFileToExamine), faAnyFile, SearchRec);
     if inRetval = 0 then
       I1 := SearchRec.Size
     else
       I1 := -1;
   finally
     SysUtils.FindClose(SearchRec);
   end;
   Result := IntToStr(I1);
 end;



 procedure TForm1.Button1Click(Sender: TObject);
 begin
   if OpenDialog1.Execute then
     label1.Caption := Get_File_Size(Opendialog1.FileName, True);
 end;

 {*******************************************************************************}

 function Get_File_Size3(const FileName: string): TULargeInteger;
 // by nico 
var
   Find: THandle;
   Data: TWin32FindData;
 begin
   Result.QuadPart := -1;
   Find := FindFirstFile(PChar(FileName), Data);
   if (Find <> INVALID_HANDLE_VALUE) then
   begin
     Result.LowPart  := Data.nFileSizeLow;
     Result.HighPart := Data.nFileSizeHigh;
     Windows.FindClose(Find);
   end;
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   if (OpenDialog1.Execute) then
     ShowMessage(IntToStr(Get_File_Size3(OpenDialog1.FileName).QuadPart));
 end;

 {*******************************************************************************}

 function Get_File_Size4(const S: string): Int64;
 var
   FD: TWin32FindData;
   FH: THandle;
 begin
   FH := FindFirstFile(PChar(S), FD);
   if FH = INVALID_HANDLE_VALUE then Result := 0
   else
     try
       Result := FD.nFileSizeHigh;
       Result := Result shl 32;
       Result := Result + FD.nFileSizeLow;
     finally
       CloseHandle(FH);
     end;
 end;