Believe it or not, some art lovers hold this to be a stupid question.
But not I. So consider a simple model and imagine the rest. You are an artist and you have better and worse ideas, as defined by either marketplace success or critical acclaim (or both). You can, to some degree, allocate your ideas across different size canvases. Some ideas only work well in the small, and some ideas only work well in the large, but still there is some flexibility. You are most likely to allocate your best ideas to the most saleable medium. And since larger pictures usually sell for more than smaller ones, why not put your better ideas into the larger pictures? You won’t waste a tremendous idea on a mere snippet of work, except perhaps as a practice or draft. The marginal revenue product (or "marginal critical acclaim product") will be higher for the bigger pictures. Of course we assume that the substitution effect outweighs the income effect. (Micro question: what assumptions about costs do we need? Does it suffice to assume that, given the cost of producing ideas, we can produce larger paintings at less than proportional cost? If you are Ellsworth Kelly, doubling the canvas size just isn’t that big a deal…)
There are caveats. If the picture is too large, and cannot hang above a sofa, perhaps it sells for less. So throw out monotonicity. You will put your best ideas into the most saleable medium, which does not always mean "bigger."
Longer songs are not better than shorter songs. I’ve never paid attention to all of "Nantucket Sleigh Ride." But the best songs will be close to around three minutes long, the dominant size or "medium" for hit songs. Songwriters and composers won’t put their best ideas into snippets. The best movies will be around two hours long, rather than a skit. Some artists will break these patterns for personal reasons; Peter Jackson wanted a three-hour King Kong for the (ha-ha) sake of the story. This may be a case of the income effect weighing in and financing self-indulgence.
Books should be better than short stories. Again, put your better ideas into the better-paying medium. Of course if customers use length as a signal of quality, these tendencies will be further strengthened. Intermediaries, such as networks, record companies, and your agent, will help enforce the constraints.
And how long are the best blog posts? The best comments to your wife? The best flirtations? The best comments on blog posts?
Thanks to Robin Hanson and Ilia Rainer for useful discussions of this point, and to Ilia for the question.