python .append method returns none when used a function

I want to write a code that will calculate the length of a list and then append the value to the end of the list. Here is what I did:

def append_size(lst):
  length=len(lst)
  newlist = lst.append(length)
  return newlist

print(append_size([23, 42, 108]))

and I receive "none" in the console. Any ideas?