Very hard to keep in mind when you are revising, and you know your story is just on the brink of the Unpublishable Void. With short stories the drop-off to the story that can't be sold on basis of length is gradual, but it has its break points -- 8000 words, for instance. Very annoying to be just at the edge of it and trying to ignore it because the story really does need some more description, or exposition, or character development.
Or to be at the other brink, and feeling the temptation to leave in a bit of rambling because the story is just over 75,000 words and would be a hard sell under it. Even when you remind yourself that you are only one third of the way through revising the novel and you may have plenty of space ahead where expansion is needed to keep things clear and dramatic.
And then for works that go over the other end, you have the short problem with the knowledge that there may be room for contraction later. . . . .