code: def size(self) -> Optional[int]:
      return self._size
  
file_path: mars/mars/storage/filesystem.py
imports: import os
  import uuid
  from typing import Dict, List, Optional, Tuple
  from ..lib.aio import AioFilesystem
  from ..lib.filesystem import FileSystem, get_fs
  from ..serialization import AioSerializer, AioDeserializer
  from ..utils import mod_hash, implements
  from .base import StorageBackend, ObjectInfo, StorageLevel, register_storage_backend
  from .core import StorageFileObject
label_desc: Use image node_id 3 for calling the FileSystemStorage obj's underlying member method code with example usage: obj.size() and returns: self