注意函数返回值类型是Int64，如果文件存在则返回文件大小，否则返回0。

function FileSize(FileName: string): Int64;
var
  sr: TSearchRec;
begin
  if FindFirst(FileName, faAnyFile, sr) = 0 then
    Result := Int64(sr.FindData.nFileSizeHigh) shl 32 + Int64(sr.FindData.nFileSizeLow)
  else
    Result := 0;

  FindClose(sr);
end;

由此可以得到获取文件夹大小的函数如下：

function FolderSize(FolderName: string): Int64;
var
  sr: TSearchRec;
begin
  Result := 0;

  if RightStr(FolderName, 1) <> '\' then FolderName := FolderName + '\';

  if FindFirst(FolderName + '*.* ', faAnyFile, sr) = 0 then
    repeat
      if (sr.Name <> '.') and (sr.Name <> '..') then begin
        Result := Result + FileSize(FolderName + sr.Name);

        if (sr.Attr and faDirectory) <> 0 then
          Result := Result + FolderSize(FolderName + sr.Name + '\');
      end;
    until FindNext(sr) <> 0;

  FindClose(sr);
end;

参考：

1. File Size - Get the Size of a File in Bytes using Delphi
   http://delphi.about.com/od/delphitips2008/qt/filesize.htm