code: func (ts *Store) Size(id string) (int64, error) {
  	sz, err := fileutil.DirSize(ts.GetPath(id))
  	if err != nil {
  		return -1, errwrap.Wrap(errors.New("error calculating size"), err)
  	}
  	return sz, nil
  }
docs: // Size returns the size of the rootfs for the provided id. It is a relatively
  // expensive operation, it goes through all the files and adds up their size.