I think folks really overestimate this. The old magazine standard of 1,000–7,500 words definitely shaped the idea of a 'short story,' but the online world has completely blown the doors off. On platforms like Royal Road or in digital magazines, I've seen pieces labeled as short stories that are barely a flash fiction at 500 words, and others that sprawl out to 15,000 words and still get called a short. The definition gets stretched thin by serialization, too—a 'short story' arc might be 30,000 words split over six updates.
Honestly, I wouldn't put a number on it. You have to look at the specific community. Literary magazines publishing online might stick closer to the traditional range, while web novel sites have a completely different scale. Reader expectations shift wildly depending on where you click.
I tried collecting a bunch for a reading challenge last year, and my spreadsheet was a mess. The average I calculated was meaningless because the sample was all over the place.