code: def expected_bfw_size(n_size):
      """
      Calculates the number of nodes generated by a single BFW for a single root node.
      :param n_size: <list> The number of neighbours at each depth level
      :return: The size of the list returned by a single BFW on a single root node
      """
      total = []
      for i, d in enumerate(n_size):
          if i == 0:
              total.append(d)
          else:
              total.append(total[-1] * d)
      return sum(total) + 1
  
file_path: stellargraph/tests/data/test_breadth_first_walker.py
imports: import pandas
  import pytest
  import numpy
  from stellargraph.data.explorer import SampledBreadthFirstWalk
  from stellargraph.core.graph import StellarGraph, StellarDiGraph
  from ..test_utils.graphs import create_test_graph, tree_graph, example_graph_random, weighted_tree
label_desc: Use image node_id 1 for calling a global function with example usage: expected_bfw_size(n_size) and returns: unknown