In my experiments with type annotations, we recently encountered this exact same issue (variables for which either sets or lists should be valid). My initial solution was almost exactly what you propose here: we wrote a SizedIterable class, simply inheriting from Sized and Iterable.

Ultimately, we didn't find this very satisfying, because we realized that strings are SizedIterables (they're sequences, too), and we really didn't want to allow accidentally passing strings that would be interpreted as iterables of characters. Unfortunately, I don't think there's any good way in Python to specify "any sized iterable that isn't a string".