def f(x: Sized) -> Whatever: ...
And, since sized objects have a __len__() method it seemed sensible to use x.__len__(). It was a good question about the use of special methods.
My advice is to avoid using the special methods in general. Use them only when defining classes that need to behave like Python objects.
(I'll make an exception for using x.__dict__, to avoid having to introduce an explicit dictionary object when there's one built-in to most objects.)
Use len(x) and be happy. The function wrapper around a special method is a common Python feature; it occurs in many places; use it.