What’s the Right Length?

When I was younger, I would have said that you should write what needs to be written, and stop there. My thinking was, at least in part, a rationalization for writing shorter pieces, which generally take less time and effort than longer work. And it was also partly a disdain for padding out papers with BS. I had a strong sense that for any question, the answer only had so much to it.
As I’ve gotten more comfortable writing, however, I find that most ideas are so entangled with others that it is almost impossible to draw any concrete line where you can say “this is all that needs to be said,” because pretty much every answer will lead to new questions.
Nowadays, I feel almost the opposite: if you have interesting material, then the question is not so much about what needs to be said, as it is about what your audience will bear. What’s the attention span of your audience? Whatever it may be, if you have a sense of it—whether they’ll read 100 words or 1,000 or 10,000—then you can give your audience a presentation that suits the time they have available.
There is a lower limit on this, however: If you are interested in discussing complex ideas, you’re probably going to need hundreds or thousands of words. Very short forms simply do not allow for tremendous detail.
This little essay (if it can be dignified as such) is about 250 words—a good length for stopping, I think. What do you think should determine how much you write: what you have to say, or the amount your reader wants?