Shorter stories that people can read during the course of a wank get more eyeballs and higher scores. They just do. I notice a very substantial dropoff when I go beyond 4k words, and I usually try to stay below that, or closer to 3k if I can, which I find to be a good balance between appealing to a broad swath of readers while still exposing some sense of story progression, character development and illumination of internal feelings.
If the story I want to tell needs more than 3-4k (which it often does) and it's not a competition entry, I find it almost always better to split into chapters.
UPDATE 10/5: see Parts One and Two of Daphne's Halloween (in my sig) for an example. This started as a monolithic story, but expanded to over 6k with all of the emotional development I wanted to include, not to mention all of the hot orgy sex. Since it has a natural breakpoint in the approximate middle anyway, it works a whole lot better as two chapters IMO.