From John McPhee’s Draft №4:
Writing is selection. Just to start a piece of writing you have to choose one word and only one from more than a million in the language. Now keep going. What is your next word? Your next sentence, paragraph, section, chapter? Your next ball of fact. You select what goes in and you decide what stays out. At base you have only one criterion: If something interests you, it goes in — if not, it stays out. That’s a crude way to assess things, but it’s all you’ve got. Forget market research. Never market-research your writing. Write on subjects in which you have enough interest on your own to see you through all the stops, starts, hesitations, and other impediments along the way.
Ideally, a piece of writing should grow to whatever length is sustained by its selected material — that much and no more.