By "interesting size" I mean "large enough that it's no longer convenient to page through it." Generally, for less than that, sorting's not necessary because you can just scan through the list visually, and for longer than that, the sort generally becomes useless anyway. It didn't seem to matter if the list was 100 long or 100,0000 long.
We saw less than 10% of users ever employing the sort controls; that's not zero, but it's low enough that you can't depend on them to cover use cases.
The problem with your examples are that you're not really showing "sorts" from a user standpoint. What a user is really doing is effectively filtering for the "show me the N least difficult", not "sort by difficulty, ascending." Also, sorting on a relatively scaled value make more sense, but a lot of the time the values in a column don't have that kind of relationship. What's the "least zip code" good for?
We saw less than 10% of users ever employing the sort controls; that's not zero, but it's low enough that you can't depend on them to cover use cases.
The problem with your examples are that you're not really showing "sorts" from a user standpoint. What a user is really doing is effectively filtering for the "show me the N least difficult", not "sort by difficulty, ascending." Also, sorting on a relatively scaled value make more sense, but a lot of the time the values in a column don't have that kind of relationship. What's the "least zip code" good for?