how to get the size of a directory(total size of all files under this directory or its subdir or ...)

There is also a function in the Internal` context (for now) that does that

Internal`DirectoryByteCount[$InstallationDirectory]

(* 6794576686 *)

which is in agreement with what my OS reports for that directory

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An estimate can be obtained without resorting to the underlying OS.

FileByteCount can be used to obtain the size of a file and FileNames can list files to any depth so combining these in to a function:

recursiveFolderSize[folder_String] := 
 Plus @@ Quiet[
   FileByteCount /@ FileNames["*", folder, Infinity] /. $Failed -> 0, 
   FileByteCount::fdir]

FileByteCount gives a message and returns $Failed for directories so the function handles these cases.

Also note that directories themselves take up disk space so the total returned will be an underestimate.

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