<human>:  get file size after opening.
<bot>: Here is a Python function to get file size after opening the file

import os

def get_file_size(filename):
  """Returns the size of the file in bytes."""
  with open(filename, 'rb') as f:
    return os.fstat(f.fileno()).st_size

# Enter the filename
filename = "test.txt"

# Calling the function
print((get_file_size(filename)))

#Example Output
# 0

#Remember to inspect code for bugs.