size = 0
for name, st in scandir(path):
    if st.st_mode is None or st.st_size is None:
        st = os.stat(os.path.join(path, name))
    if stat.S_ISREG(st.st_mode):
        size += st.st_size

It would be safer to use dir_fd parameter when supported, but I don't
think that os.scandir() should care of this problem. An higher level
API like pathlib, walkdir & cie which should be able to reuse *at() C
functions using dir_fd parameter.