function GetFileSize(FName: string): Int64;
var
ch: File;
OldMode: Integer;
begin
OldMode := FileMode;
Result := -1;
if not FileExists(FName) then Exit;
try
AssignFile(ch, FName);
FileMode := 0;
Reset(ch,1);
Result := FileSize(ch);
finally
CloseFile(ch);
FileMode := OldMode;
end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var i:integer;
begin
 i:=GetFileSize('c:/example.xls');
 showmessage(inttostr(i)+' байт');
end;