What I mean by "span of control" is the story size (and shape) that I can hold in my head and manage organically, to produce a competent (or better than competent) first draft with a strong, consistent voice. As opposed to work outside my span of control, which tends to be far more laborious, requiring multiple restarts and editing sessions and considerable editorial/critical input. Painful, in short.
Note that I have always written longer works than my span of control -- "Into the Gardens of Sweet Night", which is 15,600 words long, was written about the time that my span of control was around 3,500 words, and people liked it enough to land it on the Hugo ballot. Rocket Science is well outside even my current span of control, let alone where I was when I wrote it, but it seems to have made quite a good impression.
So obviously I don't need to write within my span of control to create a successful story. It's a distinction of craft or technique more than a quality barrier. In that case, why do I think it matters?
Two things. One, my voice is much more natural and stronger, at least in my judgment, when I'm working inside my span of control. That's where writing is fun, with rhythms and the odd shot of blistering white fire.
Two, outside my span of control I fall prey to many common writer fears and troubles. Inside my span of control, I tend to be highly confident. This means I can make more progress on areas needing improvement (emotionally engaging character arcs, for example), and play to my strengths without being self-conscious or contrived about it.
Prior to "Death of a Starship", my longest span of control has been about 20,000 words. I never left that zone through the 44,700 words of this first draft. Essentially doubled it in the last ten days. This means my perspective on my next two novel projects is much improved. Novels have never felt like a natural length to me, "natural" apparently equating roughly to "falling within my span of control."
Also, a couple of people commented on my remark about matociquala working without a net. What I meant by that specifically was that she exposed her immediate and interim goals publicly and reports daily on her progress. I work daily, or almost so, and I write from beginning to end without a lot of formal structure or process -- it's not that I really work with a net either. It's just that matociquala does in a tent with hundreds of lj readers looking on.
Any comments on "span of control"? You guys have your own experiences of equivalent phenomena? I'm curious, because by and large I am an unconscious writer...I deliberately don't break down my process in detail, I don't read a lot of books on writing...I let Fred do it down in the plumbing, and feed him tools when and how I can. "Span of control" is one of the few critical concepts I've always been overtly aware of in my own development as a writer, which gives it special resonance for me. As always, your mileage may vary.